feat(coder): v2.6 Phase 3 — lifecycle hardening (idle evict, crash recovery, worktree reaper)

Idle TTL eviction per (chat,agent) + LRU cap (never a busy backend); pure lifecycle-decisions.ts (TDD). Crash recovery lifts openchamber's health-monitor + busy-aware-restart + stale-grace state machine into opencode-server.ts (+ port reclaim) and warm-acp.ts; opencode crash -> fresh sessions, ACP -> re-session/new. F.1 turn-guard + U.6 usage preserved (their tests pass). Orphan worktree reaper (1h grace, superset-style dirty/unpushed preflight, Paseo soft-delete) + close hooks + diff re-baseline after apply_pending. 35 new tests + DB-opt-in reconnect test; 215 coder tests pass; tsc + build clean. Completes v2.6. Follow-ups out of scope: apps/server close-hook caller, 3.7 DiffPanel staging hint, live smokes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/coder/src/services/agent-backend.ts

@@ -99,4 +99,21 @@ export interface AgentBackend {
   dispose(): Promise<void>;
   /** Liveness for health endpoint + dispatcher fallback decision. §2 */
   health(): 'up' | 'down';
+  /**
+   * v2.6 Phase 3: true iff a turn is in flight on this backend. The pool's idle
+   * eviction + LRU cap NEVER evict a busy backend (design §6 busy rule); the
+   * health-monitor defers a restart while busy (stale-grace). Optional so the
+   * Phase-0 scaffold and any test double stay compatible — absent ⇒ treated as
+   * not busy. opencode-server (multi-session) is busy iff ANY session has an
+   * active turn; warm-acp (single session) iff its one slot is active.
+   */
+  isBusy?(): boolean;
+  /**
+   * v2.6 Phase 3: optional proactive health probe + busy-aware self-restart, run
+   * by the pool's periodic sweep. The opencode-server backend implements it
+   * (detects a hung-but-not-exited server and restarts when non-busy). Backends
+   * with no long-lived shared process (warm-ACP recovers lazily on its own child
+   * exit) can omit it. Must never throw — the sweep ignores rejections.
+   */
+  tickHealth?(now?: number): Promise<void>;
 }