TUI: collaboration mode UX + always submit UserTurn when enabled (#9461)

- Adds experimental collaboration modes UX in TUI: Plan / Pair
Programming / Execute.
- Gated behind `Feature::CollaborationModes`; existing behavior remains
unchanged when disabled.
- Selection UX:
- `Shift+Tab` cycles modes while idle (no task running, no modal/popup).
- `/collab` cycles; `/collab <plan|pair|pp|execute|exec>` sets
explicitly.
- Footer flash after changes + shortcut overlay shows `Shift+Tab` “to
change mode”.
  - `/status` shows “Collaboration mode”.
- Submission semantics:
- When enabled: every submit uses `Op::UserTurn` and always includes
`collaboration_mode: Some(...)` (default Pair Programming).
  - Removes the one-shot “pending collaboration mode” behavior.
- Implementation:
- New `tui/src/collaboration_modes.rs` (selection enum/cycle, `/collab`
parsing, resolve to `CollaborationMode`, footer flash line).
- Fallback: `resolve_mode_or_fallback` synthesizes a `CollaborationMode`
when presets are missing (uses current model + reasoning effort; no
`developer_instructions`) to avoid core falling back to `Custom`.
  - TODO: migrate TUI to use `Op::UserTurn`.

codex-rs/tui/src/slash_command.rs

@@ -24,6 +24,7 @@ pub enum SlashCommand {
     Fork,
     Init,
     Compact,
+    Collab,
     // Undo,
     Diff,
     Mention,
@@ -57,6 +58,7 @@ impl SlashCommand {
             SlashCommand::Status => "show current session configuration and token usage",
             SlashCommand::Ps => "list background terminals",
             SlashCommand::Model => "choose what model and reasoning effort to use",
+            SlashCommand::Collab => "change collaboration mode (experimental)",
             SlashCommand::Approvals => "choose what Codex can do without approval",
             SlashCommand::ElevateSandbox => "set up elevated agent sandbox",
             SlashCommand::Experimental => "toggle beta features",
@@ -99,6 +101,7 @@ impl SlashCommand {
             | SlashCommand::Exit => true,
             SlashCommand::Rollout => true,
             SlashCommand::TestApproval => true,
+            SlashCommand::Collab => true,
         }
     }