feat(review): fan multi-skill workers out into parallel single-skill runs

A worker configured with N skills previously joined them into one child's prompt: skills executed sequentially (or blended), so selecting more skills made the user wait for the SUM of their durations. Measured live: a two-skill claude worker ran ~9 minutes as one child.

explodeSkillWorkers splits each multi-skill worker into one worker per skill at plan time (keys like claude-code:review, deduped against existing workers), so skills run concurrently as ordinary slots — the wait becomes the slowest skill. Two exploded workers also mean the judge consolidates per-skill reports, extending the crew+judge value prop to single-agent multi-skill profiles.

--agent now selects ALL of that agent's workers as a filtered crew (previously an ambiguity error), running the single-agent path only when exactly one worker matches.

Same-agent SAME-model exploded workers defeat the existing agent+model session matching (assignments could cross, attributing tokens and transcripts to the wrong skill). AgentRun gains Skills, propagated through planned runs and both run paths, and the matcher requires skill-set agreement when both sides carry skills — mirroring the same-agent different-model disambiguation from #1313.

Verified end-to-end with a claude shim: a two-skill profile spawns two one-skill children in parallel (~3s wall for both) plus the judge.