Vouch propose from pr

/vouch-propose-from-pr

A PR is a decision: someone proposed a change, the team accepted it. The "why" gets compressed into the merge message and forgotten. This command preserves the why as cited claims in the KB.

Steps:

1. Parse $ARGUMENTS as a PR URL or <owner>/<repo>#<num>; default to the most-recently-merged PR you authored.
2. Fetch the PR title, body, and the merged-commit SHA via gh.
3. Register the merge commit as a kb_register_source so subsequent claims can cite it.
4. Read the diff. For each behavioural change (not formatting / renaming), draft one kb_propose_claim whose text summarises the new invariant the code now upholds, citing the source from step 3.
5. Propose at most five claims per PR. If a PR is that big, suggest the contributor split it next time.

Do not auto-approve. The KB's review gate is intentional; this command only fills the queue.