Session mode is lost when loadSession falls back to new session

[z-x-yang]

Summary

acpx can lose session mode (for example plan) across reconnects when session/load fails and runtime falls back to session/new.

Observed behavior:

• set-mode plan reports success
• next command reconnects, loadSession fails (Resource not found / empty-session load failure)
• acpx creates a fresh ACP session
• effective mode silently reverts to default (auto)

Why this happens

Mode switching is applied to the current ACP session id. When reconnect creates a fresh ACP session, there is no replay of the user's intended mode.

Reproduction

1. Ensure a persistent session.
2. Run set-mode plan.
3. Trigger reconnect where session/load falls back to session/new.
4. Run another config command (for example set reasoning_effort high) and inspect configOptions.mode.currentValue.

Expected: mode remains plan.
Actual: mode may be auto.

Proposed fix

Persist a desired mode at the acpx session-record level and replay it when a fresh ACP session is created:

• Persist record.acpx.desired_mode_id on successful:
  • set-mode <mode>
  • set mode <value>
• In connectAndLoadSession, if fallback creates a fresh ACP session, call session/set_mode(desired_mode_id) before continuing.
• Keep behavior best-effort: replay failures should warn in verbose mode but not hard-fail normal command flow.

Validation

Added regression coverage for fallback + replay in CLI tests:

• set plan -> reconnect fallback -> verify mode still plan
• set back to auto -> reconnect fallback -> verify mode still auto