chore: submit action to GitHub Marketplace at GA

[bennyderickm]

Context

sengol-io/sengol-github-action is public and has a valid action.yml.
The action is usable today via uses: sengol-io/sengol-github-action@v0.1.0.

Marketplace listing is deferred to GA so the listing reflects a stable,
versioned release rather than a pre-release build.

Pre-conditions before submitting

• score and report_url outputs wired (tracked in sengol-io/sengol Phase 2d issue)
• v1.0.0 release tag created on this repo
• All acceptance criteria on the SDK side closed

Steps at GA

1. Go to https://github.com/marketplace/new
2. Select sengol-io/sengol-github-action
3. Category: Security (primary) + Testing (secondary)
4. Use the README intro paragraph as the listing description
5. Tag v1.0.0 and publish the GitHub Release before submitting
6. Verify action.yml branding: icon: shield, color: blue ✅ already set

Acceptance criteria

• Action listed at https://github.com/marketplace/actions/sengol-ai-governance
• v1.0.0 release tag created on sengol-io/sengol-github-action
• Listing category: Security + Testing
• score and report_url outputs wired before submission