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>

mcdoc.toml.example

@@ -0,0 +1,19 @@
+[server]
+listen_addr = ":8080"
+
+[gitea]
+url             = "https://git.wntrmute.dev"
+org             = "mc"
+webhook_secret  = "change-me"
+poll_interval   = "15m"
+fetch_timeout   = "30s"
+max_concurrency = 4
+
+[gitea.exclude_paths]
+patterns = ["vendor/", ".claude/", "node_modules/", ".junie/"]
+
+[gitea.exclude_repos]
+names = []
+
+[log]
+level = "info"