feat(backends): add codex/gemini external runners and wire backend selection

README.md

@@ -316,6 +316,21 @@ models:
 
 Each tier can optionally specify `auth_mode` (`auto` | `api_key` | `oauth`) to control whether Flynn uses API keys vs OAuth/token auth for that provider. `use_oauth: true` remains supported as a compatibility alias for `auth_mode: oauth`.
 
+### Agent Backends
+
+Flynn can run with the built-in native backend or delegate message processing to external CLI backends.
+
+```yaml
+backends:
+  native: { enabled: true }
+  codex: { enabled: false, path: /usr/local/bin/codex }
+  claude_code: { enabled: false, path: /usr/local/bin/claude }
+  opencode: { enabled: false, path: /usr/local/bin/opencode }
+  gemini: { enabled: false, path: /usr/local/bin/gemini }
+```
+
+If multiple external backends are enabled, Flynn selects the first in this order: `codex` -> `claude_code` -> `opencode` -> `gemini`.
+
 ### Native Audio Support
 
 Voice messages from channels can be handled in two ways: