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/model-routing.yml

@@ -0,0 +1,23 @@
+version: 1
+
+# Sequential role model hints for Pi. The parent session changes model before
+# each stage when a different model is desired. No parallel agents/subagents.
+roles:
+  leader:
+    model: default
+    reasoning: medium
+  architect:
+    model: strong
+    reasoning: medium
+  implementer:
+    model: default
+    reasoning: medium
+  reviewer:
+    model: strong
+    reasoning: low
+  security:
+    model: strong
+    reasoning: medium
+  qa:
+    model: default
+    reasoning: low