Add DeepSeek V4 Flash model and coder composite via airouter

- models/airouter-deepseekv4flash.yaml: DeepSeek-V4-Flash on airouter.ch endpoint,
  temp 1.0 / top_p 1.0 per recommended reasoning defaults, 262k context / 65k output
- model-registry/airouter-deepseekv4flash.yaml: deepseek provider, complexity 9,
  creativity 9, cost_efficiency 10 (covered by airouter sub)
- harnesses/contexts/deepseek-code-methodology/v1: DeepSeek-specific methodology
  CLAUDE.md; same rules as qwen-code-methodology but with correct model header
- harnesses/composites/code-airouter-deepseekv4flash-repo/v1: coder composite
  using airouter/v1 context (shared endpoint + secret — no new ESO resources needed)

harnesses/composites/code-airouter-deepseekv4flash-repo/v1/harness.yaml

@@ -0,0 +1,15 @@
+kind: composite
+name: code-airouter-deepseekv4flash-repo
+version: 1
+description: "Code agent with Airouter DeepSeek V4 Flash + repo clone via SSH"
+
+# Inherits airouter context's label gate — only the airouter dispatcher
+# advertises this composite (see dispatcher/poller._collect_supported_harnesses).
+requires_labels: [airouter]
+
+layers:
+  - context: deepseek-code-methodology/v1
+  - context: best-practices/v1
+  - context: airouter/v1
+  - context: gitea-ssh/v1
+  - context: agent-repo/v1