docs(engine): document required server:engine channel allowlist flag

[AmberCXX]

Summary

• Spell out that claude --dangerously-load-development-channels server:engine is required for engine notifications to be delivered. Without it, the MCP subprocess connects fine and the scheduler fires on time, but every notification is silently dropped — easy to mistake for "engine doesn't work" when the root cause is a missing launch flag.
• Show the multi-channel form for users running engine alongside hub: server:hub server:engine (space-separated, not comma).
• Add a verification recipe using engine_add_task with a 30-second delay so users can confirm the channel notification path end-to-end.
• Cross-link Anthropic's Channels Reference and forge-launcher (which handles the flag automatically).

Motivation

Real debug session that triggered this: ~1 week of "engine sometimes works, sometimes doesn't" symptoms turned out to be a single missing server:engine token in the launching claude command. claude mcp list showed ✓ Connected the whole time, which strongly suggested the engine MCP was fine — but the trigger log was the only evidence anything fired, because notifications were dropped by Claude Code's research-preview allowlist. The README didn't mention this flag at all, so users following it would hit the same wall.

Scope

Docs-only — touches forge-engine/README.md only.

Test plan

• git -c core.quotepath=false status --short --untracked-files=all → only forge-engine/README.md modified
• secret grep grep -rE '@im\.wechat|ou_[a-f0-9]{16,}|sk-ant-[a-zA-Z0-9_-]{20,}|sk-[a-zA-Z0-9_-]{20,}|[0-9]{9,10}:[A-Za-z0-9_-]{35}' → no hits in changed file
• All commands and links in the new section verified to match the current repo (no fictional CLI commands; claude mcp add --scope user, --dangerously-load-development-channels server:<name>, engine_add_task all real)
• Doesn't change any code paths — no harness / self-test needed per CONTRIBUTING.md guidance for docs-only changes

🤖 Generated with Claude Code

[LinekForge]

感谢这个 PR！填了一个很实际的坑——channel notification 被静默丢弃这个行为确实很容易误判为"engine 不工作"。

整体内容准确，几个小建议：

1. claude mcp add 命令缺少 -- 分隔符

当前：

claude mcp add --scope user engine /Users/<you>/.bun/bin/bun /absolute/path/to/forge-engine/engine-channel.ts

官方文档要求 options 在 name 前，name 和 command 之间用 -- 分隔（参考 https://code.claude.com/docs/en/mcp ）。建议改为：

claude mcp add --transport stdio --scope user engine -- /Users/<you>/.bun/bin/bun /absolute/path/to/forge-engine/engine-channel.ts

2. 验证 recipe 的代码块风格

当前的 bash 代码块全是注释（# ...），没有可执行命令。README 里其他代码块都是可以直接复制执行的。建议改成一句普通文字说明，比如：

验证方法：在 Claude Code 窗口里让 agent 调用 engine_add_task（设 delay_seconds: 30）。30 秒后收到 <channel source="engine" ...> 消息说明白名单生效；收不到就检查是否漏了 server:engine。

这样和文档整体风格更一致。

3. 其他

• [!IMPORTANT] callout 用得恰当
• step 编号正确（4→5）
• 链接都验证过了，没问题
• 多 channel 的空格分隔写法 server:hub server:engine 也确认正确（hub-app/WebViewBridge.swift 里已有同样写法）

改完这两处就可以合了。

— Forge