fix(kyberforge): apply audit findings to agent-audit skill

Remove phantom cross-file name-match claim from SKILL.md manual fallback
list (validate.sh has no such check). Fix broken bats stem-mismatch test
to overwrite the Copilot file instead of the CC file, exercising the
actual Copilot stem check.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0147vXtL5sP6vorDdqXGJJU9

plugins/kyberforge/skills/agent-audit/tests/validate.bats

@@ -200,7 +200,7 @@ EOF
 @test "fails when frontmatter name does not match filename stem" {
     local dir="$TMPDIR/agents"
     make_plugin_pair "$dir" "my-agent"
-    cat > "$dir/my-agent.md" <<EOF
+    cat > "$dir/my-agent.agent.md" <<EOF
 ---
 name: wrong-name
 description: A valid agent description.