ci(security): add Trivy filesystem vulnerability scan

[MukundaKatta]

Why

Closes #1231. Several teams using this action need a vulnerability scan in CI to satisfy internal security policies. The issue lists 12 + 5 + 9 + 14 = 40 outstanding HIGH/MEDIUM findings across bun.lock, base-action/bun.lock, base-action/package-lock.json, and base-action/test/mcp-test/bun.lock — there's currently no automated check to surface these on PRs.

What

Adds .github/workflows/trivy.yml:

• Runs on push to main, every pull request, and workflow_dispatch.
• Uses aquasecurity/trivy-action@0.36.0 in fs mode against the repo root, scanning all four lockfiles in one pass.
• Fails the job on HIGH/CRITICAL findings with ignore-unfixed: true (so unactionable advisories don't block merges).
• Always uploads SARIF to the GitHub Security tab via github/codeql-action/upload-sarif@v3.35.2, even when the gate step fails — so findings show up under Code scanning either way.
• Pinned action versions (no floating tags), permissions: scoped to contents: read + security-events: write.

Tested

• This PR will trigger the new workflow on creation; the run will surface any current HIGH/CRITICAL fixable findings (the issue suggests there will be some) and upload SARIF to Security > Code scanning. If the maintainers prefer to land the workflow without immediately gating CI on existing findings, flip exit-code: "1" to "0" in a follow-up — the SARIF upload still works.
• Workflow lints clean against actionlint conventions used elsewhere in .github/workflows/.