extract Output trait: decouple AI core from Telegram

- Add src/output.rs with Output trait and 3 implementations:
  TelegramOutput (streaming via draft/edit), GiteaOutput (comments),
  BufferOutput (for worker/tests)
- Refactor run_openai_with_tools and execute_tool to use &mut dyn Output
- Remove run_claude_streaming, invoke_claude_streaming, run_openai_streaming
  (dead code — only OpenAI-compatible backend is used now)
- Remove BackendConfig::Claude code path from handler
- stream.rs: 790 → 150 lines

Cargo.lock

@@ -51,6 +51,17 @@ dependencies = [
  "syn 1.0.109",
 ]
 
+[[package]]
+name = "async-trait"
+version = "0.1.89"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9035ad2d096bed7955a320ee7e2230574d28fd3c3a0f186cbea1ff3c7eed5dbb"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.117",
+]
+
 [[package]]
 name = "atomic-waker"
 version = "1.1.2"
@@ -1045,6 +1056,7 @@ name = "noc"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "axum",
  "base64 0.22.1",
  "chrono",