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).

docs/DEPLOYMENT.md

@@ -256,9 +256,9 @@ DASHBOARD_DB_PATH=/var/lib/agenthub/dashboard.db
 DASHBOARD_WORKFLOW_POLL_MS=12000        # Workflow-tool run journals
 DASHBOARD_CODEX_POLL_MS=15000           # Codex rollout journals → codex lane running/idle + Codex sessions/tokens
 CODEX_HOME=                             # where Codex keeps sessions/ (default ~/.codex)
-DASHBOARD_ANTIGRAVITY_POLL_MS=15000     # agy conversation files → Antigravity sessions/tokens
+DASHBOARD_ANTIGRAVITY_POLL_MS=15000     # agy conversation files → Antigravity sessions/tokens (60 s while idle)
 AGENTHUB_GEMINI_DIR=                    # where agy keeps its data (default ~/.gemini)
-DASHBOARD_CURSOR_POLL_MS=15000          # Cursor agent transcripts → Cursor sessions (no tokens)
+DASHBOARD_CURSOR_POLL_MS=15000          # Cursor agent transcripts → Cursor sessions (no tokens; 60 s while idle)
 AGENTHUB_CURSOR_DIR=                    # where Cursor keeps its data (default ~/.cursor)
 AGENTHUB_AGY_USAGE=                     # 0 never runs `agy --print /usage` for the Usage panel's Antigravity quota (default on; running agy may update it)