feat: add local Recipe Oven

[nicksong-z]

Summary

Implements #78 with an additive, opt-in local Oven for Recipe workspaces.

• adds gw oven bake, reconcile, status, and clean
• adds gw create NAME --recipe FILE --oven
• claims ready slots without fetching and preserves cold Recipe creation as the fallback for clean misses
• keeps prepared dependencies at immutable physical paths and exposes claimed workspaces through named symlinks
• uses a private atomic inventory with bounded bake/claim/cleanup lifecycle recovery
• enforces reciprocal workspace/slot ownership, source-repository allowlisting, branch compare-delete, and fail-closed tamper checks
• keeps scheduling external: cron, launchd, or systemd may invoke idempotent reconciliation

Safety behavior

• incomplete bakes are never visible as ready
• quarantined or mismatched slots block claiming rather than falling back cold
• claims write workspace state last
• interrupted and partial claims are conservatively rolled back or quarantined
• partial deletion persists progress and is retryable
• normal status, sync, run, go, delete, and doctor paths verify Oven ownership
• rename/add/remove remain unsupported for Oven-backed workspaces
• credentials are not persisted in inventory; common residue files block readiness

Verification

• PATH="$(go env GOPATH)/bin:$PATH" just check
• PATH="$(go env GOPATH)/bin:$PATH" staticcheck ./...
• focused Oven race tests
• just e2e — 135/135 passed
• independent correctness and security review with all blocking findings addressed

Notes

This intentionally does not add a daemon, scheduler, generic artifact cache, CI framework, or hosted Oven. Performance and caching decisions remain in #79.

Closes #78