fix(server): alias Grok's product-name model slug to a valid ACP modelId - #7819

💭 The diff looks small, mostly model alias changes, tests, and a mock agent, without touching Effect service definitions. I want to check whether model.ts in the contracts package defines an Effect service, and verify how resolveGrokAcpBaseModelId and related functions work.

1. git_tools — MODEL_SLUG_ALIASES_BY_PROVIDER|GROK_DRIVER_KIND

2. browse_code — packages/contracts/src/model.ts

💭 This is just data and constants, not an Effect service concern. I notice GrokAcpSupport.ts falls back to "grok-build" on line 82, but I need to check whether normalizeModelSlug now maps that to grok-4.6 instead.

3. browse_code — apps/server/src/provider/acp/GrokAcpSupport.ts

💭 I'm checking that the fallback default still resolves correctly since normalizeModelSlug aliases grok-build to grok-4.6, satisfying the test expectation. This diff doesn't touch any Effect service conventions—the changed files are just the mock agent script and the test file. Since there are no actual findings, I need to produce a success result, but the output format requires structured JSON rather than a plain "All clear" string. I'll set the state to success and the summary to reflect that everything checks out clean.