chore: harden release workflow branch cleanup guidance

[tryweb]

Summary

• Add a mandatory Phase 6.5 branch-cleanup verification gate to release-workflow skill so release branches cannot silently remain after squash merge.
• Document expected squash topology and add a reset-vs-rebase recovery decision table to reduce false divergence handling.
• Sync docs/DEVELOPMENT_WORKFLOW.md with the same release safeguards and replace stash-first dirty-tree advice with safer release guidance.

Validation

• docker compose build --no-cache && docker compose up -d
• docker compose exec opencode-dev npm run typecheck
• docker compose exec opencode-dev npm run build