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

harness/contracts/evidence.schema.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://json-schema.org/draft/2020-12/schema",
-  "$id": "https://arnes.local/evidence.schema.json",
+  "$id": "https://orquestra.local/evidence.schema.json",
   "title": "Gate Evidence",
   "type": "object",
   "required": ["feature_id", "agent", "verdict", "summary", "evidence", "timestamp"],