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/codex_app_server_protocol.schemas.json

@@ -8326,7 +8326,8 @@
           "CONFIG",
           "SKILLS",
           "PLUGINS",
-          "MCP_SERVER_CONFIG"
+          "MCP_SERVER_CONFIG",
+          "SESSIONS"
         ],
         "type": "string"
       },
@@ -10772,15 +10773,20 @@
       "MigrationDetails": {
         "properties": {
           "plugins": {
+            "default": [],
             "items": {
               "$ref": "#/definitions/v2/PluginsMigration"
             },
             "type": "array"
+          },
+          "sessions": {
+            "default": [],
+            "items": {
+              "$ref": "#/definitions/v2/SessionMigration"
+            },
+            "type": "array"
           }
         },
-        "required": [
-          "plugins"
-        ],
         "type": "object"
       },
       "ModeKind": {
@@ -13545,6 +13551,27 @@
         ],
         "type": "string"
       },
+      "SessionMigration": {
+        "properties": {
+          "cwd": {
+            "type": "string"
+          },
+          "path": {
+            "type": "string"
+          },
+          "title": {
+            "type": [
+              "string",
+              "null"
+            ]
+          }
+        },
+        "required": [
+          "cwd",
+          "path"
+        ],
+        "type": "object"
+      },
       "SessionSource": {
         "oneOf": [
           {