sanitize mcp schemas for Gemini; fix gpt-codex-mini alias; add matrix filter

Gemini's generateContent API rejects arktype's `{anyOf:[{enum:[...]}]}` string-enum
encoding, `$schema` metadata, and `anyOf` with sibling fields. Port the old
sanitizer back as an isolated module (action/mcp/geminiSanitizer.ts) and gate it
on `isGeminiRouted(ctx)` so non-gemini routes see the original schema. Wires
`resolvedModel` onto ToolContext so the sanitizer can see the upstream specifier.

Also bumps `openai/gpt-codex-mini` alias from the deprecated `codex-mini-latest`
to `gpt-5.1-codex-mini`, matching the openrouter resolve.

Adds a `filter` workflow_dispatch input + MATRIX_FILTER env that restricts the
models-live matrix to aliases matching a substring, so we can iterate on a
single provider (e.g. `filter=gemini`) without paying to run every model.

mcp/server.ts

@@ -149,6 +149,10 @@ export interface ToolContext {
   jobId: string | undefined;
   mcpServerUrl: string;
   tmpdir: string;
+  // resolved upstream model specifier (e.g. "google/gemini-3.1-pro-preview").
+  // undefined when payload.proxyModel is set or when the alias is unresolvable.
+  // used by the schema sanitizer to detect Gemini-routed traffic.
+  resolvedModel: string | undefined;
 }
 
 const mcpPortStart = 3764;