feat: microVM isolation per role with a validated runbook (0.20.0)

[naashw]

Summary

• Every runner role (runbook scan, dev, checks, review, gitops ship) runs in a disposable Microsandbox microVM behind SandboxDriver; .codesema/runbook.json is validated by a real replay (codesema runbook scan) and reported to the hub (contract 0.11.0: runbook scans, runbook state, ticket verifications, changed_files on merge).
• Real end-to-end run findings folded in (see CHANGELOG 0.20.0), including the root cause of the zombie / FOREIGN KEY failures: the driver now stops the native sandbox instance before removing it and never rewrites the runtime store under the live SDK connection (scrub once per process before first use, and after exit).
• The checks package cache volume is mounted atomically (namedWith(name, 'ensure-exists')).

Verification

• CLI suite 3645/3645, typecheck, oxlint, knip, duplication, spell, arch gates green.
• Real microVM run on Microsandbox 0.6.15: scan, 3 tasks dev → checks → review_ok in one runner serve, 0 zombie, 0 store error.

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