feat: include agent metadata in workspace list responses (#27934)

Closes #27933. Related: #27897 (single-agent GET).

Agent metadata is only readable via a per-agent watch stream, so reading
it across N workspaces costs N+1 requests. This adds a batch read to the
list endpoint:

```text
GET /api/v2/workspaces?q=param:"pool=demo" include_agent_metadata:task_status
```

- New `include_agent_metadata` search key, repeatable and key-scoped. It
expands the response, it does not filter workspaces.
- `GetWorkspaces` aggregates the requested keys as JSON behind a `CASE`:
without opt-in the response is unchanged and the subquery never runs.
Runs only for the returned page, inside the same authorized query.
- Agents in the response gain `metadata`
(`[]codersdk.WorkspaceAgentMetadata`, `omitempty`), mapped by the
`workspace_agent_id` each element carries. The collection script is
omitted; it can be long.
- `codersdk.WorkspaceFilter` gains `IncludeAgentMetadata []string`.
- No wildcard, no schema change, no migration.

---

Authored by Coder Agents on behalf of @Emyrk.

docs/reference/api/agents.md

@@ -624,6 +624,23 @@ curl -X GET http://coder-server:8080/api/v2/workspaceagents/{workspaceagent} \
   ],
   "logs_length": 0,
   "logs_overflowed": true,
+  "metadata": [
+    {
+      "description": {
+        "display_name": "string",
+        "interval": 0,
+        "key": "string",
+        "script": "string",
+        "timeout": 0
+      },
+      "result": {
+        "age": 0,
+        "collected_at": "2019-08-24T14:15:22Z",
+        "error": "string",
+        "value": "string"
+      }
+    }
+  ],
   "name": "string",
   "operating_system": "string",
   "parent_id": {