vouch-propose-from-prlisted

# /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.