feat: add slog structured logging

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -31,6 +31,8 @@ The server listens on `:8080` by default. Override via `configs/config.yaml` or
 | `CODESPACE_WRITE_TIMEOUT` | HTTP write timeout (e.g. `15s`) |
 | `CODESPACE_IDLE_TIMEOUT` | HTTP idle timeout (e.g. `60s`) |
 | `CODESPACE_MAX_HEADER_BYTES` | Max header bytes (e.g. `1048576`) |
+| `CODESPACE_LOG_LEVEL` | Log level (`debug`, `info`, `warn`, `error`) |
+| `CODESPACE_LOG_FORMAT` | Log format (`json`, `text`) |
 
 ## Test
 
@@ -53,10 +55,30 @@ workspace:
   root: "./workspaces"
 process:
   opencodeCommand: "opencode"
+log:
+  level: "info"
+  format: "json"
 ```
 
 The server is started through an explicit `http.Server{Handler: router}` — it does not use `gin.Engine.Run()`.
 
+## Logging
+
+codespace uses the Go standard library `log/slog` for application logs. Default output is JSON to stdout.
+
+```yaml
+log:
+  level: "info"
+  format: "json"
+```
+
+Environment overrides:
+
+- `CODESPACE_LOG_LEVEL` (`debug`, `info`, `warn`, `error`)
+- `CODESPACE_LOG_FORMAT` (`json`, `text`)
+
+All application logs must go through `log/slog`. Do not use `fmt.Print*`, `log.Printf`, `gin.Logger()`, or third-party logging libraries in server runtime code.
+
 ## API examples
 
 Health check: