feat(coder): v2.6 Phase 0 — AgentBackend foundations (no behavior change)

Schema, interface, and service scaffold for v2.6 persistent agent sessions.
Nothing in this batch alters runtime behavior.

- schema.sql: add session_worktrees (one shared worktree per session, FK
  sessions(id)) and agent_sessions (one backend session per (session, agent),
  with backend/status CHECKs); add pending_changes.agent column for DiffPanel
  attribution. All three statements idempotent (IF NOT EXISTS).
- services/agent-backend.ts: AgentBackend interface + AgentSessionHandle,
  EnsureSessionOpts, PromptCtx, TurnResult, and the normalized transport-agnostic
  AgentEvent union (text/reasoning/tool_call/tool_update/commands). Types only.
- services/agent-pool.ts: lazy get-or-create AgentPool keyed by
  `${sessionId}:${agent}` + shared `agentPool` singleton. Empty in Phase 0.
- index.ts: widen onClose to await dispatcher.stop() then agentPool.dispose()
  (pool empty, so dispose() is inert).

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

apps/coder/src/index.ts

@@ -33,6 +33,7 @@ import { registerProviderRoutes } from './routes/providers.js';
 import { registerWebSocket } from './routes/ws.js';
 // Phase 4: dispatcher + agent probe
 import { createDispatcher } from './services/dispatcher.js';
+import { agentPool } from './services/agent-pool.js';
 import { probeAgents } from './services/agent-probe.js';
 import { getProviderSnapshot, persistProbedModels } from './services/provider-snapshot.js';
 import { setPermissionHooks } from './services/permission-waiter.js';
@@ -178,7 +179,12 @@ async function main() {
   // Phase 4: dispatcher — polls tasks table and runs inference
   const dispatcher = createDispatcher({ sql, inference: inferenceApi, broker, log: app.log, config });
   dispatcher.start();
-  app.addHook('onClose', () => dispatcher.stop());
+  app.addHook('onClose', async () => {
+    // stop() first so in-flight dispatcher turns settle, then drain the pool.
+    // Pool is empty in Phase 0 (nothing spawns yet) — dispose() is inert.
+    await dispatcher.stop();
+    await agentPool.dispose();
+  });
 
   // Register routes
   registerMessageRoutes(app, sql, broker, inferenceApi);