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/ClientRequest.json

@@ -781,6 +781,36 @@
       ],
       "type": "object"
     },
+    "FsUnwatchParams": {
+      "description": "Stop filesystem watch notifications for a prior `fs/watch`.",
+      "properties": {
+        "watchId": {
+          "description": "Watch identifier returned by `fs/watch`.",
+          "type": "string"
+        }
+      },
+      "required": [
+        "watchId"
+      ],
+      "type": "object"
+    },
+    "FsWatchParams": {
+      "description": "Start filesystem watch notifications for an absolute path.",
+      "properties": {
+        "path": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            }
+          ],
+          "description": "Absolute file or directory path to watch."
+        }
+      },
+      "required": [
+        "path"
+      ],
+      "type": "object"
+    },
     "FsWriteFileParams": {
       "description": "Write a file on the host filesystem.",
       "properties": {
@@ -4000,6 +4030,54 @@
       "title": "Fs/copyRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "fs/watch"
+          ],
+          "title": "Fs/watchRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/FsWatchParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Fs/watchRequest",
+      "type": "object"
+    },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "fs/unwatch"
+          ],
+          "title": "Fs/unwatchRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/FsUnwatchParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Fs/unwatchRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {