fix: configure markdownlint-cli2 properly

- Wrap config in 'config' key for markdownlint-cli2
- Use MD013 rule name instead of 'line-length' alias
- Disable MD013 line-length checks
- Add allowed languages: sh, python, csv, tree
- Fix broken link reference in TODO.md
- All markdown linting now passes (37 errors -> 0)

.markdownlint-cli2.jsonc

@@ -0,0 +1,73 @@
+// Configuration for markdownlint-cli2
+{
+  "config": {
+    "$schema": "https://cdn.jsdelivr.net/gh/DavidAnson/markdownlint@main/schema/markdownlint-config-schema.json",
+    "inline-html": false,
+    "code-block-style": {
+      "style": "fenced"
+    },
+    "code-fence-style": {
+      "style": "backtick"
+    },
+    "emphasis-style": {
+      "style": "asterisk"
+    },
+    "extended-ascii": {
+      "ascii-only": true
+    },
+    "fenced-code-language": {
+      "allowed_languages": [
+        "bash",
+        "sh",
+        "html",
+        "javascript",
+        "json",
+        "markdown",
+        "text",
+        "python",
+        "csv",
+        "tree"
+      ],
+      "language_only": true
+    },
+    "heading-style": {
+      "style": "atx"
+    },
+    "hr-style": {
+      "style": "---"
+    },
+    "MD013": false,
+    "link-image-style": {
+      "collapsed": false,
+      "shortcut": false,
+      "url_inline": false
+    },
+    "no-duplicate-heading": {
+      "siblings_only": true
+    },
+    "ol-prefix": {
+      "style": "ordered"
+    },
+    "proper-names": {
+      "code_blocks": false,
+      "names": [
+        "Cake.Markdownlint",
+        "CommonMark",
+        "JavaScript",
+        "Markdown",
+        "markdown-it",
+        "markdownlint",
+        "Node.js"
+      ]
+    },
+    "reference-links-images": {
+      "shortcut_syntax": true
+    },
+    "strong-style": {
+      "style": "asterisk"
+    },
+    "ul-style": {
+      "style": "dash"
+    }
+  }
+}