Add archify skill (v2.14), pi-mermaid ext (v0.3.0), project-diagramming-mermaid + project-diagramming-archify skills

- skills/archify: vendored tt-a1i/archify 2.14.0 (MIT), pure Node 18+, zero deps
- extensions/pi-mermaid: renders mermaid blocks as ASCII in TUI; deps beautiful-mermaid + mermaid
- skills/project-diagramming-*: orchestration skills, per-project docs/ convention

extensions/pi-mermaid/package.json

@@ -0,0 +1,48 @@
+{
+  "name": "pi-mermaid",
+  "version": "0.3.0",
+  "description": "🧜‍♀️ Pi extension that renders Mermaid diagrams as ASCII in the TUI",
+  "author": "Gurpartap Singh",
+  "license": "MIT",
+  "publishConfig": {
+    "access": "public"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/Gurpartap/pi-mermaid.git"
+  },
+  "bugs": {
+    "url": "https://github.com/Gurpartap/pi-mermaid/issues"
+  },
+  "homepage": "https://github.com/Gurpartap/pi-mermaid",
+  "keywords": [
+    "pi",
+    "pi-extension",
+    "pi-package",
+    "mermaid",
+    "ascii",
+    "tui"
+  ],
+  "engines": {
+    "node": ">=20"
+  },
+  "files": [
+    "index.ts",
+    "README.md",
+    "LICENSE",
+    "package.json"
+  ],
+  "dependencies": {
+    "beautiful-mermaid": "^1.0.0",
+    "mermaid": "^11.0.0"
+  },
+  "peerDependencies": {
+    "@mariozechner/pi-coding-agent": "*",
+    "@mariozechner/pi-tui": "*"
+  },
+  "pi": {
+    "extensions": [
+      "./index.ts"
+    ]
+  }
+}