feat: add verified Playwright repair loop

[tobybgy-lsd]

Summary

• add failure-doctor heal for one exact Playwright locator replacement
• require an eligible diagnosis, unambiguous source target, backup, and real rerun
• keep the patch only after exit code 0 and an unchanged patched source
• automatically roll back failed or unlaunchable verification commands
• refocus the public README and default CLI on diagnosis plus verified repair

This is intentionally conservative. It does not infer arbitrary production selectors, bypass challenges, or report an edit as a successful repair.

Verification

• python -m pytest -q --tb=short: 653 passed, 4 skipped, 333 subtests passed
• python -m build --wheel --sdist --outdir tmp\release-heal-final: passed
• python -m twine check tmp\release-heal-final\agent_failure_doctor-6.3.0*: passed
• package private-content scan: 2 archives, 0 findings
• clean venv wheel installation: passed
• installed-wheel diagnose -> heal -> rerun: status=verified, exit 0

Stack

This PR is stacked on #3 (codex/v6.3-lite-ux). Merge #3 first, then this PR.