I don't like secrets just being added via environment variables. Add support for the `_FILE` environment variable convention used by Docker secrets.
When `GITEA_ACCESS_TOKEN_FILE` is set, the token is read from the file at that path (e.g. `/run/secrets/gitea_token`). Trailing newlines are stripped to handle the typical Docker secrets file format on both Linux and Windows.
Token resolution precedence (highest to lowest):
1. `--token` / `-T` CLI flag
2. `GITEA_ACCESS_TOKEN` env var
3. `GITEA_ACCESS_TOKEN_FILE` env var
Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/186
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Dennis Gaida <gitea@mail.gaida.biz>
Co-committed-by: Dennis Gaida <gitea@mail.gaida.biz>
