test(github): pin the clock for repo grant refresh tests

[viktormarinho]

Seven GitHub repo-grant tests started failing after September 8, 2026: their seeded grants expire on that date, but the refresh calls defaulted to the real clock. They returned invalid_grant before reaching permission fallback, token minting or upstream error handling. Some invalid-grant tests also passed for the wrong reason.

Pass the fixture's June 10 clock through the existing now parameter in every refresh test that omitted it. Production code and grant expiry behavior are unchanged.

This failure reproduced on unchanged main at b30f645, independently of the catalog changes in #553. The catalog PR was merged separately as requested.

Validation:

• Before: bun test github/server/lib/repo-grant.test.ts produced the same seven CI failures on both the catalog branch and unchanged main.
• After: all 34 grant tests pass; the full GitHub suite passes all 133 tests.
• bun run check github, formatting, lint and git diff --check pass. The repository type-check command filters dependency-source diagnostics; a direct tsc --noEmit reports those dependency errors. Lint has warnings in existing MCP implementations.
• Registry generation check and 19 registry/auth tests pass on the merged catalog baseline.