fix(skill-author): resolve skill-audit findings

- script: new-skill.sh now exits 0 when target already exists (idempotent
  retry-safe) instead of exit 1; --help updated to reflect narrowed error cases
- test: updated bats test to assert success and "nothing to do" output
- body: removed speculative "Extract the skill from a real task" advice
  (human-targeted, not agent-actionable)
- formatting: converted H4 headings in Step 2 to bold text (H2/H3 two-tier model)
- provenance: removed orphan agentskills-llms-txt entry from references/sources.md;
  added discovery-only comment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/kyberforge/skills/skill-author/tests/new-skill.bats

@@ -111,8 +111,9 @@ teardown() {
     assert_failure
 }
 
-@test "fails when target already exists" {
+@test "exits 0 when target already exists (no-op)" {
     mkdir -p "$DEST/my-tool"
     run bash "$SCRIPT" my-tool "$DEST"
-    assert_failure
+    assert_success
+    assert_output --partial "nothing to do"
 }