feat(tui): retire the tui2 experiment (#9640)

## Summary
- Retire the experimental TUI2 implementation and its feature flag.
- Remove TUI2-only config/schema/docs so the CLI stays on the
terminal-native path.
- Keep docs aligned with the legacy TUI while we focus on redraw-based
improvements.

## Customer impact
- Retires the TUI2 experiment and keeps Codex on the proven
terminal-native UI while we invest in redraw-based improvements to the
existing experience.

## Migration / compatibility
- If you previously set tui2-related options in config.toml, they are
now ignored and Codex continues using the existing terminal-native TUI
(no action required).

## Context
- What worked: a transcript-owned viewport delivered excellent resize
rewrap and high-fidelity copy (especially for code).
- Why stop: making that experience feel fully native across the
environment matrix (terminal emulator, OS, input modality, multiplexer,
font/theme, alt-screen behavior) creates a combinatorial explosion of
edge cases.
- What next: we are focusing on redraw-based improvements to the
existing terminal-native TUI so scrolling, selection, and copy remain
native while resize/redraw correctness improves.

## Testing
- just write-config-schema
- just fmt
- cargo clippy --fix --all-features --tests --allow-dirty --allow-no-vcs
-p codex-core
- cargo clippy --fix --all-features --tests --allow-dirty --allow-no-vcs
-p codex-cli
- cargo check
- cargo test -p codex-core
- cargo test -p codex-cli

codex-rs/core/config.schema.json

@@ -198,9 +198,6 @@
             "steer": {
               "type": "boolean"
             },
-            "tui2": {
-              "type": "boolean"
-            },
             "undo": {
               "type": "boolean"
             },
@@ -826,32 +823,6 @@
       },
       "type": "object"
     },
-    "ScrollInputMode": {
-      "description": "How TUI2 should interpret mouse scroll events.\n\nTerminals generally encode both mouse wheels and trackpads as the same \"scroll up/down\" mouse button events, without a magnitude. This setting controls whether Codex uses a heuristic to infer wheel vs trackpad per stream, or forces a specific behavior.",
-      "oneOf": [
-        {
-          "description": "Infer wheel vs trackpad behavior per scroll stream.",
-          "enum": [
-            "auto"
-          ],
-          "type": "string"
-        },
-        {
-          "description": "Always treat scroll events as mouse-wheel input (fixed lines per tick).",
-          "enum": [
-            "wheel"
-          ],
-          "type": "string"
-        },
-        {
-          "description": "Always treat scroll events as trackpad input (fractional accumulation).",
-          "enum": [
-            "trackpad"
-          ],
-          "type": "string"
-        }
-      ]
-    },
     "ShellEnvironmentPolicyInherit": {
       "oneOf": [
         {
@@ -991,62 +962,6 @@
           "default": true,
           "description": "Enable desktop notifications from the TUI when the terminal is unfocused. Defaults to `true`."
         },
-        "scroll_events_per_tick": {
-          "description": "Override the *wheel* event density used to normalize TUI2 scrolling.\n\nTerminals generally deliver both mouse wheels and trackpads as discrete `scroll up/down` mouse events with direction but no magnitude. Unfortunately, the *number* of raw events per physical wheel notch varies by terminal (commonly 1, 3, or 9+). TUI2 uses this value to normalize that raw event density into consistent \"wheel tick\" behavior.\n\nWheel math (conceptually):\n\n- A single event contributes `1 / scroll_events_per_tick` tick-equivalents. - Wheel-like streams then scale that by `scroll_wheel_lines` so one physical notch scrolls a fixed number of lines.\n\nTrackpad math is intentionally *not* fully tied to this value: in trackpad-like mode, TUI2 uses `min(scroll_events_per_tick, 3)` as the divisor so terminals with dense wheel ticks (e.g. 9 events per notch) do not make trackpads feel artificially slow.\n\nDefaults are derived per terminal from [`crate::terminal::TerminalInfo`] when TUI2 starts. See `codex-rs/tui2/docs/scroll_input_model.md` for the probe data and rationale.",
-          "format": "uint16",
-          "minimum": 0.0,
-          "type": "integer"
-        },
-        "scroll_invert": {
-          "default": false,
-          "description": "Invert mouse scroll direction in TUI2.\n\nThis flips the scroll sign after terminal detection. It is applied consistently to both wheel and trackpad input.",
-          "type": "boolean"
-        },
-        "scroll_mode": {
-          "allOf": [
-            {
-              "$ref": "#/definitions/ScrollInputMode"
-            }
-          ],
-          "default": "auto",
-          "description": "Select how TUI2 interprets mouse scroll input.\n\n- `auto` (default): infer wheel vs trackpad per scroll stream. - `wheel`: always use wheel behavior (fixed lines per wheel notch). - `trackpad`: always use trackpad behavior (fractional accumulation; wheel may feel slow)."
-        },
-        "scroll_trackpad_accel_events": {
-          "description": "Trackpad acceleration: approximate number of events required to gain +1x speed in TUI2.\n\nThis keeps small swipes precise while allowing large/faster swipes to cover more content. Defaults are chosen to address terminals where trackpad event density is comparatively low.\n\nConcretely, TUI2 computes an acceleration multiplier for trackpad-like streams:\n\n- `multiplier = clamp(1 + abs(events) / scroll_trackpad_accel_events, 1..scroll_trackpad_accel_max)`\n\nThe multiplier is applied to the stream’s computed line delta (including any carried fractional remainder).",
-          "format": "uint16",
-          "minimum": 0.0,
-          "type": "integer"
-        },
-        "scroll_trackpad_accel_max": {
-          "description": "Trackpad acceleration: maximum multiplier applied to trackpad-like streams.\n\nSet to 1 to effectively disable trackpad acceleration.\n\nSee [`Tui::scroll_trackpad_accel_events`] for the exact multiplier formula.",
-          "format": "uint16",
-          "minimum": 0.0,
-          "type": "integer"
-        },
-        "scroll_trackpad_lines": {
-          "description": "Override baseline trackpad scroll sensitivity in TUI2.\n\nTrackpads do not have discrete notches, but terminals still emit discrete `scroll up/down` events. In trackpad-like mode, TUI2 accumulates fractional scroll and only applies whole lines to the viewport.\n\nTrackpad per-event contribution is:\n\n- `scroll_trackpad_lines / min(scroll_events_per_tick, 3)`\n\n(plus optional bounded acceleration; see `scroll_trackpad_accel_*`). The `min(..., 3)` divisor is deliberate: `scroll_events_per_tick` is calibrated from *wheel* behavior and can be much larger than trackpad event density, which would otherwise make trackpads feel too slow in dense-wheel terminals.\n\nDefaults to 1, meaning one tick-equivalent maps to one transcript line.",
-          "format": "uint16",
-          "minimum": 0.0,
-          "type": "integer"
-        },
-        "scroll_wheel_like_max_duration_ms": {
-          "description": "Auto-mode fallback: maximum duration (ms) that a very small stream is still treated as wheel-like.\n\nThis is only used when `scroll_events_per_tick` is effectively 1 (one event per wheel notch). In that case, we cannot observe a \"tick completion time\", so TUI2 treats a short-lived, small stream (<= 2 events) as wheel-like to preserve classic wheel behavior.",
-          "format": "uint64",
-          "minimum": 0.0,
-          "type": "integer"
-        },
-        "scroll_wheel_lines": {
-          "description": "Override how many transcript lines one physical *wheel notch* should scroll in TUI2.\n\nThis is the \"classic feel\" knob. Defaults to 3.\n\nWheel-like per-event contribution is `scroll_wheel_lines / scroll_events_per_tick`. For example, in a terminal that emits 9 events per notch, the default `3 / 9` yields 1/3 of a line per event and totals 3 lines once the full notch burst arrives.\n\nSee `codex-rs/tui2/docs/scroll_input_model.md` for details on the stream model and the wheel/trackpad heuristic.",
-          "format": "uint16",
-          "minimum": 0.0,
-          "type": "integer"
-        },
-        "scroll_wheel_tick_detect_max_ms": {
-          "description": "Auto-mode threshold: maximum time (ms) for the first tick-worth of events to arrive.\n\nIn `scroll_mode = \"auto\"`, TUI2 starts a stream as trackpad-like (to avoid overshoot) and promotes it to wheel-like if `scroll_events_per_tick` events arrive \"quickly enough\". This threshold controls what \"quickly enough\" means.\n\nMost users should leave this unset; it is primarily for terminals that emit wheel ticks batched over longer time spans.",
-          "format": "uint64",
-          "minimum": 0.0,
-          "type": "integer"
-        },
         "show_tooltips": {
           "default": true,
           "description": "Show startup tooltips in the TUI welcome screen. Defaults to `true`.",
@@ -1246,9 +1161,6 @@
         "steer": {
           "type": "boolean"
         },
-        "tui2": {
-          "type": "boolean"
-        },
         "undo": {
           "type": "boolean"
         },