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>

deploy/mcdoc-rift.toml

@@ -0,0 +1,14 @@
+name = "mcdoc"
+node = "rift"
+version = "v0.1.0"
+
+[build.images]
+mcdoc = "Dockerfile"
+
+[[components]]
+name = "mcdoc"
+
+[[components.routes]]
+port = 443
+mode = "l7"
+hostname = "docs.metacircular.net"