Plan mode: default should preserve clear-context, not hide it

Problem

v2.1.75 hid the "clear context and implement" option by default when accepting a plan, in response to #25734 / #18523 where users were accidentally triggering it by pressing Enter too fast.

The fix punishes intentional users to protect inattentive ones. The destructive default was a real UX problem, but the solution should have been to change the default selection (e.g., default to "implement without clearing"), not to hide the option entirely behind an undocumented setting.

Who this affects

Anyone using plan mode as a deliberate workflow boundary — research/plan in one context, clear, implement with a fresh window. This is arguably the primary value of plan mode for complex tasks: the plan file captures all decisions, and implementation gets maximum context budget without carrying research artifacts.

The showClearContextOnPlanAccept setting restores the behavior, but:

1. It's not discoverable — you have to know it exists
2. It's user-scoped friction for what should be the normal flow
3. Skills/automations that rely on plan mode for context clearing broke silently with no migration path

Suggested fix

Keep the "clear context and implement" option visible by default, but change the default selection to a non-destructive option (e.g., "implement" without clearing). Users who want to clear context can still select it deliberately. Users who mash Enter get the safe default.

Alternatively, if hiding remains the default, document showClearContextOnPlanAccept in the plan mode docs and changelog so affected users can find it.