feat(epic1): add spec-writing-airouter-repo and best-practices-airouter-repo composites

Required by the Epic 1 trigger pipeline in agent-runtimes:
- spec-draft-opus@1 uses spec-writing-airouter-repo (concept→spec-draft workflow)
- review-spec-arch-opus@1 uses best-practices-airouter-repo (spec-draft→spec-reviewed-arch)
- scope-decompose-sonnet@1 uses spec-writing-airouter-repo (spec-reviewed-arch→scoped-tagged)

Both composites require the airouter label so only the airouter dispatcher claims them.

harnesses/composites/spec-writing-airouter-repo/v1/harness.yaml

@@ -0,0 +1,13 @@
+kind: composite
+name: spec-writing-airouter-repo
+version: 1
+description: "Spec writing agent with Airouter Qwen3.6 + repo clone via SSH"
+
+requires_labels: [airouter]
+
+layers:
+  - context: spec-writing/v1
+  - context: best-practices/v1
+  - context: airouter/v1
+  - context: gitea-ssh/v1
+  - context: agent-repo/v1