app-server: add filesystem watch support (#14533)

### Summary
Add the v2 app-server filesystem watch RPCs and notifications, wire them
through the message processor, and implement connection-scoped watches
with notify-backed change delivery. This also updates the schema
fixtures, app-server documentation, and the v2 integration coverage for
watch and unwatch behavior.

This allows clients to efficiently watch for filesystem updates, e.g. to
react on branch changes.

### Testing
- exercise watch lifecycles for directory changes, atomic file
replacement, missing-file targets, and unwatch cleanup

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

@@ -1,6 +1,10 @@
 {
   "$schema": "http://json-schema.org/draft-07/schema#",
   "definitions": {
+    "AbsolutePathBuf": {
+      "description": "A path that is guaranteed to be absolute and normalized (though it is not guaranteed to be canonicalized or exist on the filesystem).\n\nIMPORTANT: When deserializing an `AbsolutePathBuf`, a base path must be set using [AbsolutePathBufGuard::new]. If no base path is set, the deserialization will fail unless the path being deserialized is already absolute.",
+      "type": "string"
+    },
     "AccountLoginCompletedNotification": {
       "properties": {
         "error": {
@@ -998,6 +1002,27 @@
       ],
       "type": "object"
     },
+    "FsChangedNotification": {
+      "description": "Filesystem watch notification emitted for `fs/watch` subscribers.",
+      "properties": {
+        "changedPaths": {
+          "description": "File or directory paths associated with this event.",
+          "items": {
+            "$ref": "#/definitions/AbsolutePathBuf"
+          },
+          "type": "array"
+        },
+        "watchId": {
+          "description": "Watch identifier returned by `fs/watch`.",
+          "type": "string"
+        }
+      },
+      "required": [
+        "changedPaths",
+        "watchId"
+      ],
+      "type": "object"
+    },
     "FuzzyFileSearchMatchType": {
       "enum": [
         "file",
@@ -4394,6 +4419,26 @@
       "title": "App/list/updatedNotification",
       "type": "object"
     },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "fs/changed"
+          ],
+          "title": "Fs/changedNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/FsChangedNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Fs/changedNotification",
+      "type": "object"
+    },
     {
       "properties": {
         "method": {