fix(notifier): migrate opencode notifier to v2 routes (v0.21.11)

CodeNomad's per-workspace instance proxy was upgraded and now only
serves opencode v2 /api/* routes; the v1 /session/... routes 403, which
broke all notifications.

- Discovery: GET {api}{proxyPath}/api/session (v2 {data,cursor} envelope)
  instead of GET {base}/session; sessions filtered by location.directory
  and the max time.updated entry is picked.
- Send: POST {api}{proxyPath}/api/session/{sid}/prompt with a {text}
  body instead of POST {base}/session/{id}/message with {parts:[...]}.
- v2 returns 2xx immediately, so the 15 s read-timeout-as-delivered
  branch is kept only as a harmless safety net.
- Updated the spec to the v2 routes/body/envelope (incl. a
  directory-filter assertion) and kept AGENTS.md in sync.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "mesh-bot",
-  "version": "0.21.10",
+  "version": "0.21.11",
   "description": "",
   "author": "",
   "private": true,