feat(proposals): emit review-gated proposals from derived CLOSE facts
- New focused emitter generate/determine/derived_close_proposals.py:
- collect eligible derived (member/subset + TRANSITIVE) with entailed Derivation + SPECULATIVE.
- deterministic sort + stable dedupe key (predicate+args+derivation+structure_key).
- write proposal-only artifacts (source=derived_close_fact) to dedicated sink.
- best-effort, counts returned, safe skips.
- RuntimeConfig: review_derived_close_proposals (default False), documented.
- ChatRuntime.idle_tick: after consolidation (when flag on), call emitter best-effort (no did_work, no engine checkpoint).
- IdleTickResult: new field derived_close_proposals_emitted (additive, documented).
- No change to existing proposal_review contract or comprehension-failure sink.
- No serving/determine side-effects.
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