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/index.html

@@ -0,0 +1,14 @@
+{{define "content"}}
+<h1>Metacircular Platform Documentation</h1>
+<div class="repo-list">
+    {{range .Repos}}
+    <div class="repo-card">
+        <h2><a href="/{{.Name}}/" hx-get="/{{.Name}}/" hx-target="#content" hx-push-url="true">{{.Name}}</a></h2>
+        {{if .Description}}<p>{{.Description}}</p>{{end}}
+        <span class="doc-count">{{len .Docs}} document{{if ne (len .Docs) 1}}s{{end}}</span>
+    </div>
+    {{else}}
+    <p>No repositories found.</p>
+    {{end}}
+</div>
+{{end}}