feat: hang detection and resumable task sessions

- runAgentSession gains an inactivity watchdog (execution.inactivityTimeoutMs,
  default 0 = off): when no session event arrives within the window, the
  session is aborted (agent abort + bash subprocess kill) with a clear
  "inactivity timeout" error
- agent sessions persist to .ralpi/sessions/*.jsonl; resume reopens the
  JSONL via SessionManager.open so an interrupted task continues with its
  prior conversation instead of restarting from scratch
- progress.json tracks sessionFile per task (persisted at session creation,
  so kill/reload mid-run is resumable); the first attempt after resume
  reuses it, failover retries stay fresh; corrupt/missing files fall back
  to a fresh session with a warning
- bump version to 0.6.0

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@mikefreno/ralpi",
-  "version": "0.5.0",
+  "version": "0.6.0",
   "description": "Execute tasks from task files/PRD's using DAG-based dependency resolution with persistent progress tracking",
   "keywords": [
     "pi-package",