feat: SessionLifecycleManager — auto-detect and repair dead tmux sessions

- Add internal/lifecycle/manager.go with Manager struct, Run() ticker loop
  (15s interval), EnsureAllSessions() for boot-time session creation, and
  reconcile() that recreates idle sessions and recovers working ones via
  SetFailed + CreateSession
- Add state.SetFailed() to record crash timestamp on SessionState
- Add internal/lifecycle/manager_test.go with mock tmux client and 3 tests:
  TestReconcileCreatesDeadSession, TestReconcileRecoversCrashedSession,
  TestEnsureAllSessions — all pass
- Wire lifecycle.Manager into cmd/claude-failover/main.go after state init

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -31,8 +31,8 @@ config.local.yaml
 *.swo
 .DS_Store
 
-# Runtime / state
-state/
-checkpoints/
+# Runtime / state (top-level only, not internal/state package)
+/state/
+/checkpoints/
 tmp/
 .agent-queue/