fix(investigate): Support standalone freeze hook path

[Gujiassh]

Summary

• Update gstack-investigate's Edit/Write freeze hook to try both bundled ../freeze and standalone marketplace ../gstack-freeze install paths.
• Update the Scope Lock availability check to use the same path resolution, so standalone installs can still enable the debug boundary.
• Add a focused regression test for both generated and template investigate skill docs.

Fixes #1647.

Validation

• bun test test/investigate-freeze-path.test.ts test/skill-validation.test.ts --timeout 30000
• bun run scripts/gen-skill-docs.ts --dry-run
• git diff --check