iMessage: promote BlueBubbles and refresh docs/skills (#8415)

* feat: Make BlueBubbles the primary iMessage integration

- Remove old imsg skill (skills/imsg/SKILL.md)
- Create new BlueBubbles skill (skills/bluebubbles/SKILL.md) with message tool examples
- Add keep-alive script documentation for VM/headless setups to docs/channels/bluebubbles.md
  - AppleScript that pokes Messages.app every 5 minutes
  - LaunchAgent configuration for automatic execution
  - Prevents Messages.app from going idle in VM environments
- Update all documentation to prioritize BlueBubbles over legacy imsg:
  - Mark imsg channel as legacy throughout docs
  - Update README.md channel lists
  - Update wizard, hubs, pairing, and index docs
  - Update FAQ to recommend BlueBubbles for iMessage
  - Update RPC docs to note imsg as legacy pattern
  - Update Chinese documentation (zh-CN)
- Replace imsg examples with generic macOS skill examples where appropriate

BlueBubbles is now the recommended first-class iMessage integration,
with the legacy imsg integration marked for potential future removal.

* refactor: Update import paths and improve code formatting

- Adjusted import paths in session-status-tool.ts, whatsapp-heartbeat.ts, and heartbeat-runner.ts for consistency.
- Reformatted code for better readability by aligning and grouping related imports and function parameters.
- Enhanced error messages and conditional checks for clarity in heartbeat-runner.ts.

* skills: restore imsg skill and align bluebubbles skill

* docs: update FAQ for clarity and formatting

- Adjusted the formatting of the FAQ section to ensure consistent bullet point alignment.
- No content changes were made, only formatting improvements for better readability.

* style: oxfmt touched files

* fix: preserve BlueBubbles developer reference (#8415) (thanks @tyler6204)

docs/zh-CN/help/faq.md

@@ -1008,7 +1008,7 @@ pnpm add -g clawhub
 
 **能否从 Linux 运行仅限 Apple/macOS 的 Skills**
 
-不能直接运行。macOS Skills 受 `metadata.openclaw.os` 和所需二进制文件限制，Skills 只有在 **Gateway 网关主机**上符合条件时才会出现在系统提示中。在 Linux 上，`darwin` 专用 Skills（如 `imsg`、`apple-notes`、`apple-reminders`）不会加载，除非你覆盖限制。
+不能直接运行。macOS Skills 受 `metadata.openclaw.os` 和所需二进制文件限制，Skills 只有在 **Gateway 网关主机**上符合条件时才会出现在系统提示中。在 Linux 上，`darwin` 专用 Skills（如 `apple-notes`、`apple-reminders`、`things-mac`）不会加载，除非你覆盖限制。
 
 你有三种支持的模式：