feat: support GitHub repositories in scan

[Mr-Lucky]

Summary

Add HTTPS GitHub repository support to agentguard scan.

• Support scanning repositories by URL before installation.
• Add --ref selection for branches, tags, fully qualified refs, and full commit SHAs.
• Use bounded, non-interactive Git acquisition with cleanup retries.
• Preserve existing local-directory scan behavior.
• Document GitHub repository scanning in the README and Codex guide.

Type

• Bug fix
• New feature / detection rule
• Refactoring
• Documentation

Testing

• npm run build passes
• npm test passes (604 tests)
• Manually tested the change

Related Issues

Closes #

[github-actions]

AgentGuard PR Review

1. severity: high — src/scanner/source.ts / resolveScanSource fetch path
   The new GitHub scan path allows --ref to be any branch/tag/fully qualified ref and passes the resolved ref directly to git fetch ... origin <expectedRevision>. For refs that resolve to a moving branch/tag, the code then checks out the fetched commit by SHA, but it does not re-verify that the fetched commit still matches the advertised ref at checkout time. A repo update between ls-remote and fetch can produce a scan of a different revision than requested, making results non-reproducible and potentially bypassing “scan before install” assumptions.
   Fix: fetch and verify the exact advertised ref atomically where possible, or re-run ls-remote after fetch and abort unless the resolved SHA still matches the requested ref; for reproducibility, prefer requiring a full commit SHA in security-sensitive flows.

2. severity: medium — src/scanner/source.ts / normalizeGithubRepositoryUrl and repo URL handling
   The scanner only accepts https://github.com/owner/repository and rejects other GitHub-hosted HTTPS forms (e.g. uppercase hostnames, .git with queryless alternate paths are okay, but www.github.com, enterprise GitHub, or URLs with encoded path variants are not). More importantly, the regex accepts any single path segment after the owner, so some malformed repository URLs can be normalized and sent to git before later failure, which can be used to trigger unnecessary network access and confusing error paths.
   Fix: validate the URL with new URL(), require exact github.com host, and perform stricter path segment checks before any git invocation; clearly reject unsupported GitHub Enterprise/alternate hosts up front.