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.

main.ts

@@ -343,6 +343,7 @@ export async function main(): Promise<MainResult> {
       jobId: runInfo.jobId,
       mcpServerUrl: "",
       tmpdir,
+      resolvedModel,
     };
     await using mcpHttpServer = await startMcpHttpServer(toolContext, { outputSchema });
     toolContext.mcpServerUrl = mcpHttpServer.url;