verify-before-complete

Invocation points:

• About to toggle a checkbox from [ ] to [x] via a gsd_* tool
• About to commit, push, or open a PR
• About to summarize a task or slice as complete
• About to say "tests pass", "build works", "lint clean", "fixed", "done"
• Replying to a user question with "yes it works" or similar

<core_principle> EVIDENCE BEFORE CLAIMS, ALWAYS. "I ran it earlier" is not evidence. A log from three tool calls ago is not evidence if code has changed since. The verification must have happened after the last code change, in this message, with fresh output.