Improve github.ts mutation coverage

[jmcte]

Context

Stryker mutation testing is configured and currently passes the break threshold, but src/lib/github.ts has important surviving mutants:

github.ts mutation score: 61.89%
github.ts covered mutation score: 72.05%

Survivors include pagination boundaries, response validation, job filtering, runner group matching, GHCR tag parsing, and URL/header construction.

Scope

• Review surviving and no-coverage mutants for src/lib/github.ts.
• Add tests for GitHub API pagination edge cases, malformed payload handling, non-ok responses, label fallback behavior, runner-group filtering, and GHCR image/tag parsing.
• Keep tests fully mocked at the fetch/API boundary.
• Avoid requiring real GitHub credentials.

Acceptance Criteria

• pnpm test -- test/github.test.ts passes.
• pnpm mutation-test shows a materially improved github.ts score.
• Pagination and malformed-response behavior have explicit assertions.
• Remaining survivors are documented as low-value formatting/string mutants or intentionally deferred.