fix: support legacy runtime migration during npm upgrades

[jeremymcs]

Legacy projects using .gsd-path/runtime/ failed during npm upgrades with a rollback message. The installer now checks before writing, offers explicit migration consent in the wizard, and supports migration followed by the update. Unattended upgrades print the exact recovery command. Migration preserves local edits, leaves a reviewable Git diff, and remains separate from installation rollback. Later updates preserve the selected runtime.

Installer-only changes now reuse valid host workflow receipts. Installation, migration, rollback, runtime pinning, and installed guards remain covered by the offline gate. Changes to installed skills, adapters, guard implementations, workflow runtime, host manifests, or package contents keep their existing live-check requirements. Missing or invalid evidence still blocks release, and the full matrix remains available explicitly.

Prepares 1.3.0.

Validation:

• Installer and runtime lifecycle regression coverage includes consent/cancel, dry run, local-edit protection, migration recovery, and pin preservation.
• New receipt-validator tests failed before the scope fix, passed afterward, and failed again when the rule was deliberately removed and restored.
• Mixed installer/adapter changes select only the affected host; shared workflow changes select all hosts; invalid or missing receipts remain blocking.
• The release plan validates all existing receipts with required_runs: []; no new live host runs are required.
• Independent validation passed 83 Python tests, 64 Node tests, and real terminal upgrade scenarios.
• All seven PR checks passed, including the full offline gate on Node 18 and Node 20.