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/flag/flag.go

@@ -1,5 +1,12 @@
 package flag
 
+// ServerConfig holds connection details for a named Gitea server.
+type ServerConfig struct {
+	Host     string `json:"host"`
+	Token    string `json:"token"`
+	Insecure bool   `json:"insecure,omitempty"`
+}
+
 var (
 	Host    string
 	Port    int
@@ -10,4 +17,8 @@ var (
 	Insecure bool
 	ReadOnly bool
 	Debug    bool
+
+	// Servers maps a friendly name to its connection config.
+	// Populated from the GITEA_SERVERS env var (JSON object).
+	Servers map[string]ServerConfig
 )