ci(scorecard): security policy, dependabot, CodeQL + Scorecard badge

[askalf]

Raises this repo's Scorecard baseline before its badge goes public (fleet program, same night as the signed-releases roll-out).

• SECURITY.md — private-advisory-first disclosure policy with response timeline and repo-appropriate scope (Security-Policy 0 → 10).
• dependabot.yml — weekly grouped updates (Dependency-Update-Tool 0 → 10).
• CodeQL workflow — SAST on pushes, PRs, and a weekly cron (SAST 0 → up).
• README — OpenSSF Scorecard badge linking to the scorecard.dev viewer.

Where applicable in this repo: workflow token permissions locked to contents: read at top level, actions pinned by commit SHA, npm install → npm ci, and a CI smoke test so future merged PRs count for CI-Tests.

Not addressed here (structural): Maintained (repo <90 days old), Code-Review (accrues via the fleet review bot going forward), CII (bestpractices.dev enrollment), Contributors.

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.