Add safe local extension deployment and health checks

[wolfiesch]

Summary

• Keep this PR focused on deployment and doctor checks; selector actions are in Add bounded selector actions with checked target identity #55 and reconnect handling is in Preserve task continuity across transient reconnects #56.
• Reject symlinked writable paths, atomically replace development receipts, and preserve rollback backups.
• Require an explicit reload override outside macOS before any installation mutation.
• Require a ready runtime plus an extension round-trip for extension health.

Verification

Deployment safety and rollback regressions, CLI contracts, and workspace typechecks passed. SDK build passed. Earlier combined-runtime live verification remains historical; this narrowed head is verified by its focused checks.

[omp-maintainer]

P3 — four independent feature/fix threads across 23 files prevent merge-at-glance.
Blocking: the deploy receipt follows symlinks and can overwrite arbitrary user files; the documented POSIX workflow uses a macOS-only default reload.
Should-fix: sanitize selector text in OMP rendering, make the TypeScript retry test prove a retry, and use Bun's crypto API.
Maintainer call: split deployment/doctor, selector actions, and reconnect/reconciliation into focused PRs. Thanks for the detailed implementation and verification notes.