chore: add security CI (CodeQL, gitleaks, dependency-review, Dependabot)

[abrichr]

What

Adds baseline security CI as part of the SOC 2 readiness program. These are
additive, non-breaking checks — no existing workflow or gate is modified.

Why

The SOC 2 gap assessment flagged that supply-chain/scanning controls existed
only on openadapt-flow and were missing across the other public repos. This
brings this repo up to the same baseline.

SOC 2 controls advanced

• OA-C-24 (CC7.1) — SAST + secret scanning + dependency review in CI
• OA-C-23 (CC7.1) — automated dependency updates (Dependabot)
• OA-C-28 (CC7.2) — vulnerability intake via SECURITY.md + private advisories
• OA-C-19 / OA-C-31 (CC6.8/CC8.1) — CODEOWNERS + all newly-introduced action tags pinned to commit SHAs

Notes

• All third-party actions are pinned to full commit SHAs (with a version comment).
• Secret scanning uses gitleaks-action, which is free for public repositories.
• CodeQL uses advanced setup (no default setup was enabled, so there is no conflict).
• These checks are not yet marked as required status checks — merging this PR
  does not change branch protection. Existing CI stays green.

Part of the SOC 2 readiness engineering work. Do not merge without founder review.

[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.