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)

models/airouter-deepseekv4flash.yaml

@@ -0,0 +1,16 @@
+name: airouter-deepseekv4flash
+provider: airouter
+endpoint:
+  type: self_hosted
+  base_url: "https://api.airouter.ch/v1"
+  api_key_env: AIROUTER_API_KEY
+runner: agentic
+default_flags:
+  model: "DeepSeek-V4-Flash"
+  temperature: 1.0
+  top_p: 1.0
+provider_context_ref: "airouter/v1"
+provider_secret_ref: "AIROUTER_API_KEY"
+session_capable: false
+context_window: 262144
+max_output_tokens: 65536