feat(lastcode): resume threads after local package builds

[lastobelus]

Maintenance threads currently keep a turn open while local Apple Silicon builds run, repeatedly checking logs. Add a Build Local Package Project Action that builds one explicitly selected checkpoint or revision and resumes the requesting thread with the result and artifact paths.

Selection is scoped to the requesting worktree and consumed once. The action loads the current helper from the primary checkout, checks the selected commit before and after execution, validates the returned artifact metadata, bounds captured output, and cancels its owned build process group. Repository guidance and failure notices direct recovery threads to the action and explain pending-continuation handling.

Validation: 9 focused build-action tests, 2 action-reporting tests, 19 existing local-update tests, 45 checkpoint-supervisor tests, targeted lint, scripts typecheck, and exact-commit Quick CI passed. Fixture processes exercise cancellation and output limits; no full package build or application restart was performed.

Implemented with GPT-6 Codex and GPT-5.6 Luna agents in the Codex harness.