docs: add branch hygiene commit-and-merge guidance

.github/copilot-instructions.md

@@ -108,6 +108,10 @@ Run these commands in order:
 - **AVOID**: Local Python development (pip install fails with network timeouts)
 - **AVOID**: Trying to run backend outside Docker (requires complex GDAL/PostGIS setup)
 
+### Branch Hygiene
+- **ALWAYS** commit and merge completed feature branches promptly once validation passes.
+- Do not leave finished work lingering in long-lived feature branches or worktrees.
+
 ### Expected Test Failures
 - Frontend check: 3 errors and 19 warnings expected (accessibility and TypeScript issues)
 - Backend tests: 2 out of 3 Django tests fail (API endpoint issues) - **DO NOT fix unrelated test failures**
@@ -178,4 +182,4 @@ docker compose down && docker compose up -d
 - Backend source: `backend/server/`
 - Static assets: `frontend/static/`
 - Database: Handled by Docker PostgreSQL container
-- Documentation: `documentation/` folder
+- Documentation: `documentation/` folder