feat: add bl config agent command for one-click coding agent configuration

Add `bl config agent` to configure a coding agent (Claude Code, Qwen Code,
OpenCode, OpenClaw, Hermes, Codex) to use a DashScope/ModelStudio endpoint
with a single command. Writers non-destructively merge into each agent's
local config with a timestamped backup and atomic writes.

- Field structures aligned to farion1231/cc-switch; qwen-code aligned to
  QwenLM/qwen-code source (modelProviders/security.auth keyed by protocol).
- provider id unified as `bailian-cli` (qwen-code brands via model entry
  name + BAILIAN_CLI_API_KEY, since it keys by protocol).
- Codex config.toml merged via smol-toml to preserve unrelated settings.
- Add writer unit tests + config e2e cases; regenerate skill reference.

pnpm-workspace.yaml

@@ -8,6 +8,7 @@ catalog:
   ajv: ^8.20.0
   boxen: ^8.0.1
   chalk: ^5.6.2
+  smol-toml: ^1.4.2
   tsx: ^4.23.0
   typescript: ^5
   undici: ^8.4.1