singleton-dispatch

PLN-725 single-agent dispatch for stage_11_extract_signals and stage_15_coverage_critic. After each prepare stage writes its manifest, this skill reads the status field (cache_hit / skipped / needs_agent) and, only on needs_agent, spawns one synchronous singleton Task that writes the by-convention pln725_*.json output the sibling consolidate stage consumes. Invoke from walker-contract step 6 immediately after stage_11_extract_signals or stage_15_coverage_critic finishes. Do NOT use for the reviewer fleet (stage_20 — see the spawn-reviewers skill) or the verifier fleet (stage_23 — see the verify-findings skill).