Add service layer and MCP server with read tools

Slice 1: internal/service is the one capability layer both the HTTP API and the MCP server call (AGENT.md 1.7); /api/repos and /api/repo route through it. Slice 2: internal/mcp serves an MCP server over Streamable HTTP at /mcp (go-sdk v1.8.0) with read tools list_repos and get_repo as thin adapters over the service, plus a round-trip test. Enabled air polling so hot reload works across the Docker-on-Windows bind mount.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.air.toml

@@ -12,6 +12,11 @@ tmp_dir = "tmp"
   exclude_dir = ["tmp", "bin", ".git", "repos"]
   delay = 500
   stop_on_error = true
+  # Poll for changes instead of relying on fsnotify: filesystem events do NOT
+  # cross the Windows host -> Linux container bind mount, so watch-based reload
+  # silently never fires. Polling is the reliable option in Docker on Windows.
+  poll = true
+  poll_interval = 500
 
 [log]
   time = true