[rgarcia]

Adds GitHub's workflow dependency lockfile. .github/workflows/actions.lock records the commit behind every action the workflows use. Runs are checked against it, so a moved tag on a third-party action can't change what runs in CI.

• Added .github/workflows/actions.lock, generated with gh actions-lock --no-narrow. gh actions-lock --verify passes (all 8 workflows valid).
• The generated workflows keep their SHA-pinned uses: lines exactly as emitted, so the next generator promotion doesn't conflict with this change. The lockfile is keyed by those SHAs.
• One line changes: in release-please.yml, pnpm/action-setup@f40ffcd… was the SHA of the annotated v4 tag object, not a commit, and the lockfile can't record it. It now pins pnpm/action-setup@b906aff… # v4.3.0, the commit that tag points to, so the code that runs is the same.
• Not covered: the job-level reusable workflows from kernel/security-workflows/...@main (semgrep.yml, vuln-remediation.yml). A lockfile doesn't cover job-level reusable workflow calls.

Follow-up: when the generator bumps an action pin, actions.lock has to be regenerated in the same change. Otherwise runs of that workflow fail at startup.

[rgarcia]

Superseded by kernel/kernel-python-sdk-staging#14 (https://github.com/kernel/kernel-python-sdk-staging/pull/14), which landed the same change on staging; promotion has carried it to next (a0d2e4c) and it ships with the next release.