[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}

agents/skills/v8-security-triaging/SKILL.md

diff --git a/agents/skills/v8-security-triaging/SKILL.md b/agents/skills/v8-security-triaging/SKILL.md
index 72cd961..0599dea 100644
--- a/agents/skills/v8-security-triaging/SKILL.md
+++ b/agents/skills/v8-security-triaging/SKILL.md
@@ -16,6 +16,12 @@
 - **Technical Skepticism**: Treat all reporter claims (e.g., "OOB write", "RCE",
   "Silent Write") as **hypotheses**, not facts. Your primary job is empirical
   verification.
+- **Mandatory Local Reproduction**: A report MUST NOT be classified as a
+  "Vulnerability" unless it is empirically reproduced locally (e.g.,
+  demonstrating a crash, memory corruption, or a clear security boundary
+  violation). If the provided POC does not reproduce the claimed behavior, do
+  NOT classify it as a vulnerability. Instead, report that it did not reproduce
+  in the final report.
 - **Scope Limitation**: This skill is strictly for **triaging and impact
   analysis**. It does NOT include implementing a fix or creating a CL. Fixing is
   a separate task that must be explicitly requested by the user after triage is
@@ -160,7 +166,10 @@
   (Status, Classification, Rationale, etc.) and ensure there are **double line
   breaks** between each list item for optimal rendering in Buganizer.
 - **Content**:
-  - **Classification**: Vulnerability / Bug / Not a Bug (Intended Behavior).
+  - **Classification**: Vulnerability / Bug / Not a Bug (Intended Behavior) /
+    Failed to Reproduce. **MANDATORY**: Only classify as "Vulnerability" if
+    local reproduction was successful. If reproduction fails, classify as
+    "Failed to Reproduce".
 
   - **Security Impact**: Provide the label (e.g., `Security_Impact-Head`) and a
     short explanation. Skip or simplify the CVSS vector unless requested.