chore: move to public gitea instance and drop project-local gitea MCP

- Remove the project-local gitea MCP (opencode.json) and gitea-mcp-server
  from the dev shell — the gitea MCP is now installed globally and points
  at https://gitea.edley.me
- Update README/project.md links and the flake package homepage to the
  new public instance
- The GITEA_ACCESS_TOKEN for the old homeserver is removed from
  .secrets.env (gitignored); the global token from /run/secrets is used

opencode.json

@@ -7,14 +7,6 @@
     }
   },
   "mcp": {
-    "gitea": {
-      "type": "local",
-      "command": ["gitea-mcp", "-t", "stdio"],
-      "environment": {
-        "GITEA_HOST": "http://homeserver:3050",
-        "GITEA_ACCESS_TOKEN": "{env:GITEA_ACCESS_TOKEN}"
-      }
-    },
     "obs": {
       "type": "local",
       "command": ["npx", "-y", "obs-mcp"],