[agents] Add mandatory Audit phase to security triaging skill

This CL improves the reliability of the v8-security-triaging skill by
adding a mandatory Phase 6: Verification & Self-Correction (Audit).

Key changes:
- Added a subagent-driven Audit phase to cross-reference the triage
  findings with docs/security/triaging.md before presenting them to
  the user.
- Added a Technical Quality Checklist and Common Misclassification
  Pitfalls section to the v8-security-triaging skill.
- Synced the shared agents/skills/ directory with the latest
  improvements already present in the CLI's internal skills.
- Refined conversation ID handling to avoid nonsensical values in
  triage reports.
- Added more detailed indicators for safe termination and sandbox
  protection in the v8-poc-classification skill.

R=ishell@chromium.org

No-Try: true
Change-Id: I0e6379e40f2941454916e6ad5c351dc83e9d0391
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/7827758
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#107195}