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/error.html

@@ -0,0 +1,7 @@
+{{define "content"}}
+<div class="error-page">
+    <h1>{{.Code}}</h1>
+    <p>{{.Message}}</p>
+    <a href="/" hx-get="/" hx-target="#content" hx-push-url="true">Back to index</a>
+</div>
+{{end}}