Update POST-PUSH-PROTOCOL with git diff --stat HEAD~1 pre-push blast-radius check

Update ~/shared-knowledge/POST-PUSH-PROTOCOL.md with `git diff --stat HEAD~1` pre-push check. CARRY-FORWARD from 2026-06-16 reflection (memory/2026-06-16.md, priority 2). Rule born 2026-06-15 22:30 UTC when stray SiteEnergyCharts.tsx from BOLT-1193 in-flight branch nearly shipped on BOLT-1281 commit; caught by ad-hoc `git diff --stat HEAD~1` between commit and push. Reset + re-commit + force-push fixed it. Rule fired in its own birth session. Implementation: add a "Pre-push blast-radius check" subsection to POST-PUSH-PROTOCOL.md that requires `git diff --stat HEAD~1` before `git push` whenever working on a branch touched by parallel in-flight work (stacked PRs, switched-into branches). File count must match intent. Acceptance: POST-PUSH-PROTOCOL.md has the check, all future PRs from this agent pass it. Priority 2.