fix(symphony): correct runtime model — long-lived session, with multi-session support

Earlier framing assumed each /schedule-fired tick was a cold start
needing on-disk reconciliation. The actual model is a long-lived
orchestrator session where /schedule fires ticks within the running
session — TaskList sees prior-tick tasks just fine. Reframed
accordingly, while leaving the door open to other trigger sources
(Discord channel, webhook, second user session) which DO need
cross-session reconciliation.

Reconciliation logic now has a fast path and a slow path:

- Fast path: task in our TaskList → it's ours → reconcile via TaskGet.
- Slow path: task NOT in our TaskList (another session, or our session
  restarted) → check the worker's heartbeat file. Fresh → another
  session owns it, leave alone. Stale or missing → consider abandoned,
  retry; do not call TaskStop on a task we don't own.

Workers now write .symphony/heartbeats/<issue-id>.json on start,
refresh it at heartbeat_interval_ms cadence (default 60s), and delete
it on graceful exit. Stale heartbeats are how abandonment is detected
across sessions and after crashes.

Also added session_id to state.json running entries so cross-session
reconcilers know who spawned what.

WORKFLOW.md gains heartbeat_interval_ms and heartbeat_stale_ms knobs
under agent: with documented defaults.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

symphony/templates/WORKFLOW.md

@@ -37,6 +37,11 @@ agent:
   max_retry_backoff_ms: 600000
   max_concurrent_agents_by_state:
     "in progress": 2
+  # Worker heartbeat cadence — affects how quickly other Symphony sessions
+  # (Discord triggers, webhooks, separate user sessions) detect abandoned
+  # workers. Defaults are safe; tighten if you have many concurrent sessions.
+  heartbeat_interval_ms: 60000      # workers refresh the heartbeat at most this often
+  heartbeat_stale_ms:    300000     # reconciler treats heartbeats older than this as abandoned
 ---
 
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