[agents] Mandate local reproduction for vulnerability classification

This CL updates the v8-security-triaging skill to enforce empirical
verification as a prerequisite for classifying reports as vulnerabilities.

Key changes:
- Added a "Mandatory Local Reproduction" core principle to the
  v8-security-triaging skill.
- Updated the classification guidance to mandate "Failed to Reproduce"
  if local reproduction is unsuccessful.
- Enforced that "Vulnerability" status can only be assigned after
  successful local reproduction.

R=ishell@chromium.org

No-Try: true
Change-Id: I81a86835fea75c5a0d84d4878dee74fb2efb2975
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/7830726
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#107196}