fold forge_cursor into hyperhive-harness.json + prose comments (mara/argus review)

docs/persistence.md

@@ -129,7 +129,7 @@ Consolidated harness state file written atomically (`.tmp` + rename) by
 Shape:
 
 ```json
-{ "rate_limited": false, "needs_login": false }
+{ "rate_limited": false, "needs_login": false, "active_model": "…", "forge_cursor": { "42": "2026-07-01T18:00:00Z" } }
 ```
 
 - `rate_limited` — set when the harness detects a 429 from the Claude
@@ -138,6 +138,17 @@ Shape:
 - `needs_login` — set when a turn hits 401 (expired OAuth credentials);
   cleared by `"online"` status (re-auth completed). Drives the
   `needs_login` flag alongside the `claude_has_session` check.
+- `active_model` — the resolved Claude model for the dashboard badge.
+- `forge_cursor` — the `forge_notify` delivery-dedupe cursor
+  (notification thread id → last-delivered `updated_at`), so a
+  rebuild/restart doesn't re-deliver the whole currently-unread forge
+  backlog. See [`forge.md`](forge.md).
+
+Multiple harness tasks write this file (the turn loop for the first
+three fields, the `forge_notify` poller for `forge_cursor`), so every
+writer goes read-modify-write under a shared in-process lock — each
+preserves the fields it doesn't own rather than reconstructing the file
+from scratch.
 
 hive-c0re reads this file on each `build_all` sweep (~10s) via
 `container_view::read_harness_flags`. Falls back to the legacy individual