Feat: Add Signal channel integration

[eti0]

Summary

Add Signal as a messaging channel for PicoClaw.

Motivation

Signal is a major privacy-focused messaging platform. Many users prefer it over Telegram/WhatsApp for self-hosted AI assistants due to its E2E encryption and privacy stance.

Proposed Solutions

Option A: signal-cli (Recommended)
Use signal-cli JSON-RPC API:

• Run signal-cli daemon --http as external dependency
• PicoClaw connects to local HTTP API to send/receive messages
• Pros: Mature, handles E2E encryption, minimal code to maintain
• Cons: Requires JVM (~50-100MB RAM) — documented as optional external dependency

Option B: libsignal
Use Signal's official libsignal library:

• Native Signal Protocol implementation
• Pros: No external JVM process, lower memory footprint
• Cons: More complex integration, requires maintaining crypto code

Configuration

{
  "channels": {
    "signal": {
      "enabled": true,
      "account": "+1234567890",
      "signal_cli_url": "http://localhost:8080",
      "allow_from": ["+0987654321"]
    }
  }
}

Notes

• Credentials stored in ~/.local/share/signal-cli/ (if using signal-cli)
• Supports text messages, with voice/attachments possible as follow-up

References

• signal-cli
• signal-cli-rest-api
• libsignal

[nikolasdehor]

Well-structured proposal. The signal-cli JSON-RPC approach (Option A) is the most practical path given picoclaw's architecture -- it mirrors how the WhatsApp channel uses an external bridge process.

Implementation notes:

• The channel interface in pkg/channels/ follows a consistent pattern: BaseChannel embed + Start()/Stop()/handleMessage(). A Signal channel would implement the same interface.
• The allow_from field should use FlexibleStringSlice (not raw []string) for consistency with other channels. Issue PicoClaw 代码问题清单(看代码时顺手用claude过了下，如不对请忽略) #116 (BUG-007) documents this inconsistency with the Slack channel.
• For the WhatsApp channel that just landed (PR Added a native WhatsApp channel implementation. #655, merged today), they used a native Go implementation rather than an external bridge. A similar approach using github.com/nanu-c/signal-go or similar Go bindings could avoid the JVM dependency, though signal-cli is more battle-tested.

The E2E encryption handling is the main differentiator vs. other channels -- signal-cli abstracts this entirely, which is a significant advantage for maintainability.