feat: add config file support to CLI

- Add ~/.webber/config.toml for persistent settings
- Support api.url, api.key, cli.mode, cli.stream, history.file
- Environment variables override config file values
- Add 'config' command to show settings and init config file
- Update all commands to use config defaults

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

AGENTS.md

@@ -108,9 +108,19 @@ cd webber-cli
 - **Tab completion** for commands and file paths
 - **Command history** persisted to `~/.webber_history`
 - **Session persistence** - conversations saved and resumable
+- **Config file** - persistent settings via `~/.webber/config.toml`
 - **Runtime mode switching** via `mode plan|default|auto_accept`
 - **Directory navigation** via `cd <path>`
 
+**Configuration:**
+```bash
+# Show current config
+.venv/bin/webber-cli config
+
+# Initialize config file with defaults
+.venv/bin/webber-cli config --init
+```
+
 **Note:** The API server must be running for CLI commands to work.
 
 ---