External agent session support (#19895)

## Summary

This extends external agent detection/import beyond config artifacts so
Codex can detect recent sessions files from the external agent home and
import them into Codex rollout history.

## What changed

- Added a focused `external_agent_sessions` module for:
  - session discovery
  - source-record parsing
  - rollout construction
  - import ledger tracking
- Wired session detection/import into the app-server external agent
config API.
- Added compaction handling so large imported sessions can be resumed
safely before the first follow-up turn.

## Testing

Added coverage for:
- recent-session detection
- custom-title handling
- recency filtering
- dedupe and re-detect-after-source-change behavior
- visible imported turn construction
- backward-compatible import payload deserialization
- end-to-end RPC import flow
- rejection of undetected session paths
- repeat-import behavior
- large-session compaction before first follow-up

Ran:
- `cargo test -p codex-app-server external_agent_config_import_ --test
all`

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -849,7 +849,8 @@
         "CONFIG",
         "SKILLS",
         "PLUGINS",
-        "MCP_SERVER_CONFIG"
+        "MCP_SERVER_CONFIG",
+        "SESSIONS"
       ],
       "type": "string"
     },
@@ -1779,15 +1780,20 @@
     "MigrationDetails": {
       "properties": {
         "plugins": {
+          "default": [],
           "items": {
             "$ref": "#/definitions/PluginsMigration"
           },
           "type": "array"
+        },
+        "sessions": {
+          "default": [],
+          "items": {
+            "$ref": "#/definitions/SessionMigration"
+          },
+          "type": "array"
         }
       },
-      "required": [
-        "plugins"
-      ],
       "type": "object"
     },
     "ModeKind": {
@@ -3022,6 +3028,27 @@
       ],
       "type": "string"
     },
+    "SessionMigration": {
+      "properties": {
+        "cwd": {
+          "type": "string"
+        },
+        "path": {
+          "type": "string"
+        },
+        "title": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "cwd",
+        "path"
+      ],
+      "type": "object"
+    },
     "Settings": {
       "description": "Settings for a collaboration mode.",
       "properties": {