Add goal app-server API (2 / 5) (#18074)

Adds the app-server v2 goal API on top of the persisted goal state from
PR 1.

## Why

Clients need a stable app-server surface for reading and controlling
materialized thread goals before the model tools and TUI can use them.
Goal changes also need to be observable by app-server clients, including
clients that resume an existing thread.

## What changed

- Added v2 `thread/goal/get`, `thread/goal/set`, and `thread/goal/clear`
RPCs for materialized threads.
- Added `thread/goal/updated` and `thread/goal/cleared` notifications so
clients can keep local goal state in sync.
- Added resume/snapshot wiring so reconnecting clients see the current
goal state for a thread.
- Added app-server handlers that reconcile persisted rollout state
before direct goal mutations.
- Updated the app-server README plus generated JSON and TypeScript
schema fixtures for the new API surface.

## Verification

- Added app-server v2 coverage for goal get/set/clear behavior,
notification emission, resume snapshots, and non-local thread-store
interactions.

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

@@ -3028,6 +3028,93 @@
       ],
       "type": "object"
     },
+    "ThreadGoal": {
+      "properties": {
+        "createdAt": {
+          "format": "int64",
+          "type": "integer"
+        },
+        "objective": {
+          "type": "string"
+        },
+        "status": {
+          "$ref": "#/definitions/ThreadGoalStatus"
+        },
+        "threadId": {
+          "type": "string"
+        },
+        "timeUsedSeconds": {
+          "format": "int64",
+          "type": "integer"
+        },
+        "tokenBudget": {
+          "format": "int64",
+          "type": [
+            "integer",
+            "null"
+          ]
+        },
+        "tokensUsed": {
+          "format": "int64",
+          "type": "integer"
+        },
+        "updatedAt": {
+          "format": "int64",
+          "type": "integer"
+        }
+      },
+      "required": [
+        "createdAt",
+        "objective",
+        "status",
+        "threadId",
+        "timeUsedSeconds",
+        "tokensUsed",
+        "updatedAt"
+      ],
+      "type": "object"
+    },
+    "ThreadGoalClearedNotification": {
+      "properties": {
+        "threadId": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "threadId"
+      ],
+      "type": "object"
+    },
+    "ThreadGoalStatus": {
+      "enum": [
+        "active",
+        "paused",
+        "budgetLimited",
+        "complete"
+      ],
+      "type": "string"
+    },
+    "ThreadGoalUpdatedNotification": {
+      "properties": {
+        "goal": {
+          "$ref": "#/definitions/ThreadGoal"
+        },
+        "threadId": {
+          "type": "string"
+        },
+        "turnId": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "goal",
+        "threadId"
+      ],
+      "type": "object"
+    },
     "ThreadId": {
       "type": "string"
     },
@@ -4727,6 +4814,46 @@
       "title": "Thread/name/updatedNotification",
       "type": "object"
     },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "thread/goal/updated"
+          ],
+          "title": "Thread/goal/updatedNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/ThreadGoalUpdatedNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Thread/goal/updatedNotification",
+      "type": "object"
+    },
+    {
+      "properties": {
+        "method": {
+          "enum": [
+            "thread/goal/cleared"
+          ],
+          "title": "Thread/goal/clearedNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/ThreadGoalClearedNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Thread/goal/clearedNotification",
+      "type": "object"
+    },
     {
       "properties": {
         "method": {