fix: harden worker completion checks

[Maleick]

Summary

• Auto-commit worker changes after successful opencode run before accepting terminal status.
• Reject worker-reported COMPLETE results when the diff is tests-only, marking the workspace STUCK with evidence.
• Fetch the remote base before selecting the worktree base ref to avoid stale branch dispatch.

Closes #253

Test Plan

• bash hooks/opencode/test-policy.sh
• bash -n hooks/opencode/.sh hooks/.sh
• bash hooks/opencode/smoke-test.sh