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/codex_app_server_protocol.schemas.json

@@ -883,6 +883,54 @@
           "title": "Fs/copyRequest",
           "type": "object"
         },
+        {
+          "properties": {
+            "id": {
+              "$ref": "#/definitions/v2/RequestId"
+            },
+            "method": {
+              "enum": [
+                "fs/watch"
+              ],
+              "title": "Fs/watchRequestMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/FsWatchParams"
+            }
+          },
+          "required": [
+            "id",
+            "method",
+            "params"
+          ],
+          "title": "Fs/watchRequest",
+          "type": "object"
+        },
+        {
+          "properties": {
+            "id": {
+              "$ref": "#/definitions/v2/RequestId"
+            },
+            "method": {
+              "enum": [
+                "fs/unwatch"
+              ],
+              "title": "Fs/unwatchRequestMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/FsUnwatchParams"
+            }
+          },
+          "required": [
+            "id",
+            "method",
+            "params"
+          ],
+          "title": "Fs/unwatchRequest",
+          "type": "object"
+        },
         {
           "properties": {
             "id": {
@@ -4053,6 +4101,26 @@
           "title": "App/list/updatedNotification",
           "type": "object"
         },
+        {
+          "properties": {
+            "method": {
+              "enum": [
+                "fs/changed"
+              ],
+              "title": "Fs/changedNotificationMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/FsChangedNotification"
+            }
+          },
+          "required": [
+            "method",
+            "params"
+          ],
+          "title": "Fs/changedNotification",
+          "type": "object"
+        },
         {
           "properties": {
             "method": {
@@ -7444,6 +7512,29 @@
         ],
         "type": "string"
       },
+      "FsChangedNotification": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "description": "Filesystem watch notification emitted for `fs/watch` subscribers.",
+        "properties": {
+          "changedPaths": {
+            "description": "File or directory paths associated with this event.",
+            "items": {
+              "$ref": "#/definitions/v2/AbsolutePathBuf"
+            },
+            "type": "array"
+          },
+          "watchId": {
+            "description": "Watch identifier returned by `fs/watch`.",
+            "type": "string"
+          }
+        },
+        "required": [
+          "changedPaths",
+          "watchId"
+        ],
+        "title": "FsChangedNotification",
+        "type": "object"
+      },
       "FsCopyParams": {
         "$schema": "http://json-schema.org/draft-07/schema#",
         "description": "Copy a file or directory tree on the host filesystem.",
@@ -7698,6 +7789,70 @@
         "title": "FsRemoveResponse",
         "type": "object"
       },
+      "FsUnwatchParams": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "description": "Stop filesystem watch notifications for a prior `fs/watch`.",
+        "properties": {
+          "watchId": {
+            "description": "Watch identifier returned by `fs/watch`.",
+            "type": "string"
+          }
+        },
+        "required": [
+          "watchId"
+        ],
+        "title": "FsUnwatchParams",
+        "type": "object"
+      },
+      "FsUnwatchResponse": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "description": "Successful response for `fs/unwatch`.",
+        "title": "FsUnwatchResponse",
+        "type": "object"
+      },
+      "FsWatchParams": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "description": "Start filesystem watch notifications for an absolute path.",
+        "properties": {
+          "path": {
+            "allOf": [
+              {
+                "$ref": "#/definitions/v2/AbsolutePathBuf"
+              }
+            ],
+            "description": "Absolute file or directory path to watch."
+          }
+        },
+        "required": [
+          "path"
+        ],
+        "title": "FsWatchParams",
+        "type": "object"
+      },
+      "FsWatchResponse": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "description": "Created watch handle returned by `fs/watch`.",
+        "properties": {
+          "path": {
+            "allOf": [
+              {
+                "$ref": "#/definitions/v2/AbsolutePathBuf"
+              }
+            ],
+            "description": "Canonicalized path associated with the watch."
+          },
+          "watchId": {
+            "description": "Connection-scoped watch identifier used for `fs/unwatch` and `fs/changed`.",
+            "type": "string"
+          }
+        },
+        "required": [
+          "path",
+          "watchId"
+        ],
+        "title": "FsWatchResponse",
+        "type": "object"
+      },
       "FsWriteFileParams": {
         "$schema": "http://json-schema.org/draft-07/schema#",
         "description": "Write a file on the host filesystem.",