crew: factor hub-spoke out of /crew:review into a generic primitive

[jdidion]

/crew:do's hub-spoke dispatch is advisory-only today — it routes code-review tasks back to /crew:review but can't auto-dispatch non-review hub-spoke tasks because the hub-spoke orchestration logic is embedded in the review skill.

What's embedded

skills/review/SKILL.md bakes together:

1. Hub-spoke orchestration (generic): resolve roster → spawn N external reviewers in parallel → wait → merge → report.
2. Review-specific specialization: diff capture, worktree entry for MR/PR scope, merge-findings attribution logic, post-and-monitor to GitLab/GitHub.

A generalized hub-spoke primitive would split (1) out so other skills — and /crew:do's auto-dispatch — can use it.

Proposed shape

Something like /crew:spoke <task> or as a library tool tools/spawn-roster that:

• Takes a prompt file + model roster
• Spawns each per the existing resolve-backend flow
• Waits and collects reply files
• Returns the reply-file paths (not a merged report — that's caller's job)

/crew:review then becomes "call spawn-roster with a review prompt + reviewer-specific merge."

Why this hasn't shipped

No second consumer. The abstraction cost vs. single-consumer YAGNI tips toward leaving it embedded until there's a real second user. Filing this so when that user appears (likely /crew:do hub-spoke auto-dispatch, or a new skill like /crew:plan-and-delegate), the work is pre-framed.

Originally flagged in PR #61's follow-ups list.