Persist and replay desired mode across load fallback

[z-x-yang]

Summary

Fix mode loss across reconnect when session/load falls back to session/new.

This persists the user's desired mode at the ACPX session-record layer and replays it on fresh ACP sessions created during fallback.

Changes

• Added SessionAcpxState.desired_mode_id persistence support.
• Added src/session-mode-preference.ts helpers to normalize/read/write desired mode.
• Persist desired mode after successful:
  • set-mode <mode>
  • set mode <value>
    including queue-owner control paths.
• In connectAndLoadSession, when fallback creates a fresh ACP session, replay desired_mode_id via session/set_mode.
• Added verbose diagnostics for replay success/failure.

Tests

Added/updated tests:

• test/cli.test.ts
  • set-mode persists across load fallback and replays on fresh ACP sessions
• test/session-conversation-model.test.ts
  • clone preserves desired_mode_id
• test/session-persistence.test.ts
  • persisted records preserve desired_mode_id
• test/mock-agent.ts
  • added minimal setSessionMode / setSessionConfigOption support for mode/config replay scenarios

Validation run locally:

• npm run format:check
• npm run build:test
• node --test dist-test/test/session-conversation-model.test.js
• node --test dist-test/test/session-persistence.test.js
• node --test --test-name-pattern='set-mode persists across load fallback and replays on fresh ACP sessions' dist-test/test/cli.test.js

Fixes #45.

[z-x-yang]

Related upstream Codex ACP work for full plan-mode interoperability:

• feat: allow session/set_mode to switch collaboration presets (plan/default) zed-industries/codex-acp#168 (session/set_mode collaboration presets incl. plan)
• Persist session mode across reconnect/load zed-industries/codex-acp#172 (mode persistence across load/reconnect)

This ACPX fix is still independently useful because ACPX is where reconnect fallback (load -> new) is handled and where session continuity is scoped.