Implement mcdoc v0.1.0: public documentation server

Single-binary Go server that fetches markdown from Gitea (mc org),
renders to HTML with goldmark (GFM, chroma syntax highlighting,
heading anchors), and serves a navigable read-only documentation site.

Features:
- Boot fetch with retry, webhook refresh, 15-minute poll fallback
- In-memory cache with atomic per-repo swap
- chi router with htmx partial responses for SPA-like navigation
- HMAC-SHA256 webhook validation
- Responsive CSS, TOC generation, priority doc ordering
- $PORT env var support for MCP agent port assignment

33 tests across config, cache, render, and server packages.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/templates/doc.html

@@ -0,0 +1,17 @@
+{{define "content"}}
+{{if .TOC}}
+<nav class="toc">
+    <details open>
+        <summary>Contents</summary>
+        <ul>
+        {{range .TOC}}
+            <li class="toc-h{{.Level}}"><a href="#{{.ID}}">{{.Text}}</a></li>
+        {{end}}
+        </ul>
+    </details>
+</nav>
+{{end}}
+<article class="doc-content">
+    {{.Content}}
+</article>
+{{end}}