perf: cheaper ingest — no re-parse of live hooked transcripts, delta hook writes, lighter pollers

- Session sync leaves a transcript alone while hooks feed its session live
  (active, newest event a hook's, touched within DASHBOARD_SYNC_LIVE_HOOK_MS,
  default 5 min) and catches it up once quiet; a hook-created session (which
  importSession skips anyway) is only snapshotted, never parsed.
- Hook writer keeps a per-transcript mark (applied after commit): an
  unchanged TranscriptCache result skips token/compaction/API-error/turn
  writes, a grown one walks only the entries after the last one handled.
- Codex usage: one stamp query per pass instead of one row per journal;
  between full passes (first + every 5 min) only the newest day folders and
  journals written in the last hour are statted; fork parents resolved from
  the table or a full walk.
- Cursor/Antigravity polls back off to 60 s after idle passes
  (server/lib/backoff-poll.js); lane sync runs only while another dashboard
  shares the data dir (checked every 30 s).

ARCHITECTURE.md

@@ -1066,7 +1066,7 @@ flowchart TD
 - Uses Node.js `http` module directly — no dependencies.
 - Resolution order is **env override → discovery file → default**:
   - `CLAUDE_DASHBOARD_PORT` forces a single target (no fan-out, no discovery).
-  - Otherwise `server/lib/server-info.js` reads `~/.claude/.agenthub.json`, prunes dead-PID entries, and returns one live `port` per unique `dataDir` (lowest port wins when two instances share `~/.claude/agenthub`). The handler POSTs to each returned port in parallel. The other instance learns of a lane the hooks moved through `broadcastForeignLaneChanges` (`server/routes/lanes.js`): every `DASHBOARD_LANE_SYNC_MS` (2 s) it reads `PRAGMA data_version`, which moves only on another connection's commit, and re-broadcasts each lane whose `updated_at` changed (and `removed` for a vanished one) to its own browsers. Sessions and events are not re-broadcast this way — a page on that instance shows them on its next fetch.
+  - Otherwise `server/lib/server-info.js` reads `~/.claude/.agenthub.json`, prunes dead-PID entries, and returns one live `port` per unique `dataDir` (lowest port wins when two instances share `~/.claude/agenthub`). The handler POSTs to each returned port in parallel. The other instance learns of a lane the hooks moved through `broadcastForeignLaneChanges` (`server/routes/lanes.js`): every `DASHBOARD_LANE_SYNC_MS` (2 s) — while `peersSharingDataDir()` finds another live dashboard on its data dir, which it checks every 30 s — it reads `PRAGMA data_version`, which moves only on another connection's commit, and re-broadcasts each lane whose `updated_at` changed (and `removed` for a vanished one) to its own browsers. Sessions and events are not re-broadcast this way — a page on that instance shows them on its next fetch.
   - If neither yields anything, the handler falls back to `4820`.
 - Per-target promises never reject — a dead listener can't starve the others, and the handler can wait on `Promise.all` for clean exit timing.
 
@@ -1223,7 +1223,12 @@ common "nothing changed" tick is just a handful of `stat` calls), funnels them
 through the same `parseSessionFile` + `importSession` pipeline as every other
 path, and broadcasts `session_created` for newly imported sessions /
 `session_updated` for grown ones — the same events hooks emit, so the UI
-refreshes live. All timers and watchers are `unref`'d and best-effort; nothing
+refreshes live. Two kinds of transcript are not re-parsed: one whose session
+hooks are feeding **live** (status `active`, newest event a hook's, a hook
+within `DASHBOARD_SYNC_LIVE_HOOK_MS`, default 5 min) is left until it goes
+quiet or ends — then caught up once — and one whose session a hook created
+(no `imported` flag, which `importSession` skips anyway) only has its durable
+snapshot refreshed, never a parse. All timers and watchers are `unref`'d and best-effort; nothing
 here blocks shutdown or can take down the server.
 
 ### Upload request sequence
@@ -1579,7 +1584,11 @@ re-point if that changes.
 `DASHBOARD_CODEX_POLL_MS` beat, a first pass over every journal in the
 background at startup). Each file's read offset and parse state live in the
 `codex_rollouts` table, so an unchanged file is never re-read and a grown one is
-read from where it stopped. `token_count` events are summed per `turn_context`
+read from where it stopped (a pass loads every row's size/mtime in one query).
+The first pass and one every five minutes walk every journal; the poll's other
+passes stat only the two newest day folders plus the journals the table saw
+written in the last hour (or not yet read to the end), so a thread resumed
+after an hour's quiet in an older folder is picked up by the next full pass. `token_count` events are summed per `turn_context`
 model — `input_tokens` includes `cached_input_tokens` (stored as input − cached
 plus cache_read) and `output_tokens` includes reasoning. A **fork** copies its
 parent's whole history into its own file, token events included, so every total
@@ -1654,7 +1663,9 @@ the binary's own proto descriptors.
 
 **Sessions and tokens** (`server/lib/antigravity-usage.js`,
 `startAntigravityUsagePoll`, every `DASHBOARD_ANTIGRAVITY_POLL_MS`, default
-15000, `0` disables). agy writes one SQLite file per conversation,
+15000, `0` disables; after a pass that read nothing, wrote nothing and left no
+fresh session to settle it slows to once a minute until one finds work —
+`server/lib/backoff-poll.js`). agy writes one SQLite file per conversation,
 `~/.gemini/antigravity-cli/conversations/<conversation id>.db`. Each is opened
 read-only whenever its mtime (or its non-empty `-wal`'s) changes;
 `antigravity_conversations` holds the mtime read, so a session the user deleted
@@ -1743,7 +1754,8 @@ is ever inferred.
 ## Cursor Sessions
 
 `server/lib/cursor-usage.js` (`startCursorUsagePoll`, every
-`DASHBOARD_CURSOR_POLL_MS`, default 15000, `0` disables) reads the agent
+`DASHBOARD_CURSOR_POLL_MS`, default 15000, `0` disables; idle passes slow it to
+once a minute like the Antigravity poll) reads the agent
 transcripts Cursor writes — the CLI (`cursor-agent`) and the editor's agent both —
 at `~/.cursor/projects/<project slug>/agent-transcripts/<id>/<id>.jsonl`
 (`AGENTHUB_CURSOR_DIR` overrides `~/.cursor`). Each becomes a `sessions` row with