Restore Greybeard review checklist in Corbits idiom

[TheGreatAxios]

Summary

• Greybeard carries an ordered review checklist in Corbits idiom: architectural claim, constraint ownership, holes and anti-patterns, ranked risks, hold / revise / block verdict
• The prompt frames review as analysis first: read_file, grep, and ask_director before considering a spawn, with no self-spawn language and no numeric caps or scheduler theater
• The package header records provenance (CL-7019 plus the GaaS greybeard original) and the substitution: the GaaS source at abklabs/agents plugins/gaas/agents/greybeard.md @ 6e16b6c is not available locally, so this is a Corbits-idiom restoration rather than a 1:1 copy, with the delegate-for-review shape becoming read-first plus spawn only on a concrete unknown
• maySpawn stays true with the limited intern, explorer, critic allowlist: CL-7670 has not landed on main (no spawn-block commit or docs guidance found), so the second-to-land rebase direction does not apply yet

Verification

• bun test src/agent/directors/greybeard/ --randomize --seed 424242 passes (20 pass, 0 fail)
• bun run typecheck, bun run build, and bun scripts/guard-real-projects-dir.ts pass
• Full suite bun test ./src ./tests ./evals ./scripts --randomize --seed 424242 passes (7029 pass, 0 fail)
• bun run check is green except a pre-existing oxfmt failure in src/subagent/spawn-agent-worktree.test.ts that reproduces on clean origin/main without this change

Fixes CL-7662

[linear-code]

CL-7662