feat: Orquestra - sequential orchestration runtime

- Context isolation: fresh Pi process per stage (run_stage.py)
- Gate enforcement: blocks close without approved gates
- Auto commit/push on feature close (close_feature.py)
- Write restrictions: only allowed directories (ALLOWED_WRITE_DIRS)
- Pi extension: orquestra-status with /orquestra-stage command
- Documentation: context-handoff.md, updated README
- Scripts: agent_status.py, verify.sh, install.sh updated

.gitignore

@@ -0,0 +1,8 @@
+__pycache__/
+*.pyc
+.pytest_cache/
+.codegraph/
+
+# Local Pi runtime state
+.atl/
+.env