chore(skill): Own tttk-authoring in the repo

Move the skill to .agents/skills/tttk-authoring and point gen:skill-reference at it. Activate it by symlinking ~/.agents/skills/tttk-authoring to the in-repo copy.

Extend the reference drift guard to cover the skill copy now that it is versioned, and document activation in docs/development.md.

src/cli/validate/known.test.ts

@@ -41,11 +41,11 @@ describe("known element catalog", () => {
     expect(known).toEqual(registered);
   });
 
-  test("docs/reference.md is up to date", () => {
-    const committed = readFileSync(
-      join(process.cwd(), "docs", "reference.md"),
-      "utf-8",
-    );
+  test.each([
+    ["docs/reference.md"],
+    [join(".agents", "skills", "tttk-authoring", "reference.md")],
+  ])("%s is up to date", (relativePath) => {
+    const committed = readFileSync(join(process.cwd(), relativePath), "utf-8");
 
     expect(committed).toBe(renderReference());
   });