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.

models.ts

@@ -86,7 +86,7 @@ export const providers = {
       },
       "gpt-codex-mini": {
         displayName: "GPT Codex Mini",
-        resolve: "openai/codex-mini-latest",
+        resolve: "openai/gpt-5.1-codex-mini",
         openRouterResolve: "openrouter/openai/gpt-5.1-codex-mini",
       },
       o3: {