verify: cross-verify OpenCode signal-closure

[4444J99]

OpenCode implemented validate_signal_closure (385e0b6). Verify implementation, edge cases, test coverage.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs.

[4444J99]

Engagement disposition: evolving. I checked the referenced implementation claim against current main: commit 385e0b6 is not resolvable in organvm/tool-interaction-design, and there is no validate_signal_closure symbol on main. The closest shipped verifier is conductor/cross_verify.py, which checks guardrailed handoff constraints, locked files, never-touch patterns, and convention drift. This issue still needs the actual signal-closure artifact/branch identified or reimplemented before edge-case verification can be completed.