docs(workflow): formalize gitflow process and templates

.github/PULL_REQUEST_TEMPLATE.md

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+## Summary
+
+Describe what changed and why.
+
+## Branch and Target
+
+- Source branch: `feature/*` / `release/*` / `hotfix/*`
+- Target branch: `develop` / `main`
+
+## Checklist
+
+- [ ] Scope is focused and avoids unrelated refactors
+- [ ] Docker stack runs (`docker compose up -d`)
+- [ ] Tests are passing
+- [ ] Migrations included (if needed)
+- [ ] Docs updated (if needed)
+- [ ] Changelog updated under `Unreleased`
+- [ ] No secrets committed
+
+## Testing
+
+List commands and key outputs.
+
+## Risk and Rollback
+
+Briefly note deployment risk and rollback approach.