Auto-exit Plan Mode when executing a skill

Context

Raised in Slack. Users who prefer Plan Mode as their default frequently kick off skills as new tasks. Skills almost never benefit from running in Plan Mode, they are meant to execute, not plan.

Problem

When a user is in Plan Mode and triggers a skill (e.g. /commit, /simplify), the skill runs in Plan Mode too. This means the skill describes what it would do instead of actually doing it, which is almost never the desired behavior.

Expected behavior

Two possible improvements (not mutually exclusive):

1. Prompt on discrepancy: When a skill is triggered while in Plan Mode, show a prompt like: "It looks like you're running a skill in Plan Mode. Did you mean to execute the skill?" with options to continue in Plan Mode or switch to execution.

2. Auto-exit Plan Mode for skills (preferred): Automatically exit Plan Mode when a skill is invoked, so the skill executes normally. This could be controlled by a setting (e.g. autoExitPlanModeForSkills: true) for users who want the current behavior.

The issue names no files, tests, or entry points. Start by tracing how Plan Mode state is handled when a skill such as /commit or /simplify is invoked, then determine where the mode transition should occur. Done means skill execution no longer remains in Plan Mode, while the proposed prompt or setting behavior is resolved.