feat(ai/recipes/litellm): declare chat + expansion touchpoints (#2208)

The litellm recipe shipped only an `embedding` touchpoint. `getProviderCapabilities()` in `src/core/ai/capabilities.ts` throws when `recipe.touchpoints.chat` is missing, `classifyCapabilities()` returns `'unknown'`, and `enforceSubagentCapable()` in `src/core/model-config.ts` silently falls back to `TIER_DEFAULTS.subagent` (anthropic). Any brain that routes paid traffic through a litellm-style proxy AND has no `ANTHROPIC_API_KEY` then sees every subagent loop dispatch throw `AIConfigError: Anthropic ... requires ANTHROPIC_API_KEY`. The user's explicit `models.tier.subagent = litellm:*` choice is overridden without their knowledge.

Declare `chat` and `expansion` touchpoints mirroring the openai recipe's shape: `models: []` (litellm proxies arbitrary backends; allowlist is intentionally empty, since `assertTouchpoint` already skips allowlist checks for `tier: 'openai-compat'`), `supports_tools: true`, `supports_subagent_loop: true`, `supports_prompt_cache: false` (OpenAI-compat backends don't honor Anthropic-style `cache_control`), `max_context_tokens: 200_000` (conservative GPT-5-family default; per-deployment override needed for smaller-context backends), costs `undefined` (varies by proxied provider).

Reproduction (deterministic):

1. Fresh brain with no `ANTHROPIC_API_KEY` in env.
2. `gbrain config set models.tier.subagent litellm:gpt-5.4` (or any `litellm:*` string).
3. `gbrain models` warns and falls back to `anthropic:claude-sonnet-4-6`.
4. Submit any subagent job; throws `AIConfigError: Anthropic ... requires ANTHROPIC_API_KEY`.

After the patch, `classifyCapabilities('litellm:gpt-5.4', recipe)` returns `degraded:no_caching` (chat-capable, no Anthropic-style prompt cache). `enforceSubagentCapable` no longer steals the model choice. The subagent loop emits a one-time `degraded:no_caching` warn about prompt-cache absence; cost scales linearly with conversation length, accepted trade for the proxy path.

src/core/ai/recipes/litellm-proxy.ts

@@ -41,6 +41,21 @@ export const litellmProxy: Recipe = {
       // mismatched-dim responses pre-storage).
       supports_multimodal: true,
     },
+    expansion: {
+      models: [],
+      cost_per_1m_tokens_usd: undefined,
+      price_last_verified: '2026-06-14',
+    },
+    chat: {
+      models: [],
+      supports_tools: true,
+      supports_subagent_loop: true,
+      supports_prompt_cache: false,
+      max_context_tokens: 200_000,
+      cost_per_1m_input_usd: undefined,
+      cost_per_1m_output_usd: undefined,
+      price_last_verified: '2026-06-14',
+    },
   },
   setup_hint: 'Run LiteLLM (https://docs.litellm.ai) in front of any provider; set LITELLM_BASE_URL (include the /v1 suffix if your proxy serves the OpenAI route there, e.g. http://localhost:4000/v1) + pass --embedding-model litellm:<model> and --embedding-dimensions <N>.',
 };