feat: allow session/set_mode to switch collaboration presets (plan/default)

[z-x-yang]

Why

acpx documents set-mode plan, but with codex-acp the ACP call fails with Invalid params because set_session_mode currently only accepts approval presets (read-only/auto/full-access).

In practice this means ACP clients can set approval mode, but cannot switch collaboration presets (especially plan) via session/set_mode.

What this PR changes

• Extends mode handling in src/thread.rs so session/set_mode supports both:
  1. existing approval presets (read-only, auto, full-access)
  2. collaboration presets from models_manager.list_collaboration_modes() (including plan and default)
• Exposes collaboration presets in SessionModeState.availableModes so ACP clients can discover them.
• Tracks active_mode_id so load/config mode state reflects the latest selected mode.
• Keeps existing approval-preset behavior unchanged.

Validation

• cargo check
• cargo test test_set_mode_plan_routes_to_collaboration_mode

Behavior notes

• set_mode(plan) now routes through Op::OverrideTurnContext { collaboration_mode: Some(...) }.
• set_mode(auto/read-only/full-access) still updates approval/sandbox policies exactly as before.

If maintainers prefer, I can split this into:

1. mode discovery/state changes, and
2. set_mode routing changes.

[benbrandt]

We can also expose this as a config option instead? and allow for setting both?

[z-x-yang]

@benbrandt great point — yes, this should support both paths.

In codex-acp, session/set_config_option with config_id="mode" already routes to handle_set_mode(...), so this change effectively enables both:

• session/set_mode (e.g. set-mode plan)
• session/set_config_option(mode=...) (e.g. set mode plan)

I verified both flows locally via acpx against this branch. If helpful, I can add an explicit test for the set_config_option("mode", "plan") path as a follow-up in this PR.

[z-x-yang]

Follow-up update pushed to this PR branch (7099e35):

• Added session mode persistence store at $CODEX_HOME/acp/session-mode-overrides.v1.json
• Persist mode override after successful:
  • session/set_mode
  • session/set_config_option when config_id == "mode"
• Replay saved mode on new_session / load_session after thread init
• Corruption-safe behavior: warn + continue
• Added unit tests for store persistence and corrupt-file tolerance

Related tracking issue: #172.