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/agents.matrix.yml

@@ -56,6 +56,14 @@ roles:
       - regression_checks
       - emit_qa_verdict
 
+  documenter:
+    can_edit: ["work/artifacts/", "docs/", "README.md", "HOWTO.md", "platforms/pi/README.md", "harness/contracts/"]
+    cannot_edit: ["src/", "tests/", "backlog/features.json:status"]
+    responsibilities:
+      - document_changes_when_needed
+      - update_docs_api_contracts_user_facing_notes
+      - produce_documenter_evidence
+
 anti_cheat:
   - "Implementer cannot promote feature to done"
   - "Done requires reviewer/security/qa approved artifacts"