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>

config.json

@@ -6,11 +6,12 @@
         "-t", "stdio",
         "--host", "https://gitea.com",
         "--token", "<your personal access token>"
-      ]
+      ],
       "env": {
         "GITEA_HOST": "https://gitea.com",
-        "GITEA_ACCESS_TOKEN": "<your personal access token>"
+        "GITEA_ACCESS_TOKEN": "<your personal access token>",
+        "GITEA_SERVERS": "{\"dev\":{\"host\":\"https://gitea.dev.example.com\",\"token\":\"<dev-token>\"},\"prod\":{\"host\":\"https://gitea.prod.example.com\",\"token\":\"<prod-token>\"}}"
       }
     }
   }
-}
+}