security: validate --behavioral skillName to prevent path traversal

The skillName argument from --behavioral has zero validation and is
used directly in path.join() for reading .json files, reading
SKILL.md, and WRITING .grading.json results. A value like
'../../etc/passwd' traverses outside the project directory for
both reads and writes.

Fix: Validate skillName against /^[a-z0-9]+(-[a-z0-9]+)*$/ before
any filesystem operations. This matches the existing KEBAB_CASE
validation in skill-lint.js.

scripts/run-evals.js

@@ -443,7 +443,16 @@ function parseGrading(raw) {
   return ok ? g : null;
 }
 
+// Skill name must be a valid kebab-case identifier — no path separators,
+// no "..", no absolute paths. Without this, --behavioral "../../x" would
+// resolve to files outside the project tree for both reads and writes.
+const VALID_SKILL_NAME = /^[a-z0-9]+(-[a-z0-9]+)*$/;
+
 function runBehavioral(skillName, dryRun) {
+  if (!skillName || !VALID_SKILL_NAME.test(skillName)) {
+    console.error(`Invalid skill name: "${skillName}" — must be kebab-case (e.g. "my-skill")`);
+    process.exit(1);
+  }
   const caseFile = path.join(CASES_DIR, `${skillName}.json`);
   if (!fs.existsSync(caseFile)) {
     console.error(`No eval case file for "${skillName}"`);