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.

.agents/skills/tttk-authoring/markdown.md

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+# Alerts and mermaid
+
+Standard markdown features that `tttk` renders. They are not tttk syntax — see
+`reference.md` for the element and role catalog.
+
+## GitHub-style alerts
+
+```markdown
+> [!NOTE]
+> 这是一条备注
+
+> [!TIP]
+> 提示内容
+
+> [!IMPORTANT]
+> 重要信息
+
+> [!WARNING]
+> 警告信息
+
+> [!CAUTION]
+> 危险警告
+```
+
+## Mermaid diagrams
+
+Fenced `mermaid` blocks render as diagrams:
+
+````markdown
+```mermaid
+graph TD
+    A[开始] --> B{选择}
+    B -->|选项 1| C[结果 1]
+    B -->|选项 2| D[结果 2]
+```
+````
+
+Supported: flowchart, sequenceDiagram, classDiagram, stateDiagram, erDiagram,
+pie.