feat: add first-party skills, ponytail lock, and full lint suite

- add ponytail skill set to skills-lock.json (14 locked skills total)
- add first-party medusa skill under skills/, layered on install
- make test now runs ruff, shellcheck, and markdownlint-cli2
- CI installs the lint tools before make test in both workflows
- MIT license, funding buttons, thanks and donation sections in README
- .markdownlint-cli2.jsonc config; ignore lint caches in .gitignore

.markdownlint-cli2.jsonc

@@ -0,0 +1,13 @@
+{
+  "config": {
+    "default": true,
+    "MD013": false,
+    "MD033": false,
+    "MD041": false
+  },
+  "ignores": [
+    "node_modules",
+    ".agents",
+    ".claude"
+  ]
+}