feat(channel): add Signal channel via signal-cli JSON-RPC daemon

Adds a new Signal messaging channel that connects to a running
signal-cli daemon's native HTTP API (JSON-RPC + SSE).

  [channels_config.signal]
  http_url = "http://127.0.0.1:8686"
  account = "+1234567890"
  group_id = "group_id"  # optional, omit for all
  allowed_from = ["+1111111111"]
  ignore_attachments = true
  ignore_stories = true

Implementation:
- SSE listener at /api/v1/events for incoming messages
- JSON-RPC sends via /api/v1/rpc (method: send)
- Health check via /api/v1/check
- Typing indicators via sendTyping RPC
- Supports DMs and group messages (room_id filtering)
- Allowlist-based sender filtering (E.164 or wildcard)
- Optional attachment/story filtering
- Fixed has_supervised_channels() to include signal + irc/lark/dingtalk

Registered in channel list, doctor, start, integrations registry, and
daemon supervisor gate. Includes unit tests for config serde, sender
filtering, room matching, envelope processing, and deserialization.

No new dependencies (uses existing uuid, futures-util, reqwest).

src/onboard/wizard.rs

@@ -2305,6 +2305,7 @@ fn setup_channels() -> Result<ChannelsConfig> {
         webhook: None,
         imessage: None,
         matrix: None,
+        signal: None,
         whatsapp: None,
         email: None,
         irc: None,