✨ /feature: non-interactive invocation support (structured collision skip + pre-validated scope)

[PierreBrisorgueil]

When `/feature #N` is invoked by an orchestrating agent (headless run), the skill's interactive waits block the run. Two changes, both scoped to non-interactive invocations (interactive behavior unchanged):

1. Phase 0.0 collision → structured skip. When the invoker declares a non-interactive run, a detected collision (other assignee, fresh WIP comment) must return a structured `SKIP: collision — ` result instead of a y/N prompt. Collision ≠ failure; the caller decides what to do next.
2. Phase 0 pre-validated scope mode. When the invoker states the issue body carries an already-validated scope, the "present plan & wait for user validation" step is satisfied by the issue body — proceed without waiting. Also return `STOP: ` (structured) instead of asking when the scope is insufficient.

Also: `gh` command failures in Phase 0.0 (e.g. stale auth) should surface as a structured STOP, not an open question.