feat: add multi-server support via GITEA_SERVERS env var

Allows connecting to multiple Gitea instances from a single MCP server.
Set GITEA_SERVERS as a JSON object mapping names to {host, token, insecure}
configs. Every tool gets an optional "server" parameter, and a new
"list_servers" tool is added to discover available connections.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/context/context.go

@@ -3,5 +3,7 @@ package context
 type contextKey string
 
 const (
-	TokenContextKey = contextKey("token")
+	TokenContextKey    = contextKey("token")
+	HostContextKey     = contextKey("host")
+	InsecureContextKey = contextKey("insecure")
 )