refactor: relocate outbox I/O to session-manager + dead-code sweep

## Outbox extraction (delivery.ts → session-manager.ts)

File I/O for outbound attachments now lives in session-manager.ts alongside
the symmetric inbound extractAttachmentFiles. delivery.ts no longer touches
the filesystem — it hands buffers to the adapter and calls clearOutbox on
success.

- New `readOutboxFiles(agentGroupId, sessionId, messageId, filenames)` and
  `clearOutbox(agentGroupId, sessionId, messageId)` in session-manager.ts.
- deliverMessage in delivery.ts loses ~35 lines of fs/path code and its
  `fs`/`path` imports.

## Dead-code sweep

TypeScript's --noUnusedLocals surfaced several cruft imports. Fixed:

- src/container-runner.ts: drop unused `markContainerIdle` import; drop
  unused `session` parameter from `buildContainerArgs` signature.
- src/delivery.ts: drop unused `getSession`, `writeSessionMessage`,
  `wakeContainer` imports.
- src/host-sweep.ts: drop unused `updateSession`, `outboundDbPath` imports.
- container/agent-runner/src/poll-loop.ts: drop unused `config`,
  `processingIds` params from `processQuery`.
- Test files: drop unused imports in channel-registry.test, db-v2.test,
  host-core.test.

Skipped: `conversations` state in chat-sdk-bridge.ts (never read but
tangled with public `updateConversations` method; cleaning it risks a
merge conflict with the channels branch at the next sync).

## Validation

- `pnpm run build` clean
- `pnpm test` — 137 host tests pass
- `bun test` in container/agent-runner — 17 tests pass
- Service boots (`NanoClaw running`, `OneCLI approval handler started`)
  and shuts down cleanly on SIGTERM

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

src/host-sweep.ts

@@ -10,7 +10,7 @@
 import type Database from 'better-sqlite3';
 import fs from 'fs';
 
-import { getActiveSessions, updateSession } from './db/sessions.js';
+import { getActiveSessions } from './db/sessions.js';
 import { getAgentGroup } from './db/agent-groups.js';
 import {
   countDueMessages,
@@ -21,7 +21,7 @@ import {
   retryWithBackoff,
 } from './db/session-db.js';
 import { log } from './log.js';
-import { openInboundDb, openOutboundDb, inboundDbPath, outboundDbPath, heartbeatPath } from './session-manager.js';
+import { openInboundDb, openOutboundDb, inboundDbPath, heartbeatPath } from './session-manager.js';
 import { wakeContainer, isContainerRunning } from './container-runner.js';
 import type { Session } from './types.js';