feat(channels): add native Signal channel via signal-cli HTTP daemon (#271)

* feat(channels): add native Signal channel via signal-cli HTTP daemon

Implement a native Rust Signal channel that connects to a running
signal-cli daemon's HTTP endpoint, enabling Signal messaging without
WASM overhead.

Architecture:
- SSE listener at /api/v1/events for receiving messages with automatic
  reconnection and exponential backoff
- JSON-RPC client at /api/v1/rpc for sending messages and typing
  indicators
- Reply target tracking via Arc<RwLock<HashMap>> to route responses
  back to the correct DM or group conversation

Features:
- User allowlisting supporting E.164 phone numbers, bare UUIDs, and
  uuid:-prefixed identifiers (matching OpenClaw's format)
- Group allowlisting with wildcard (*) support
- Configurable story and attachment-only message filtering
- Health check via signal-cli /api/v1/check
- Broadcast support to all tracked reply targets

Configuration via environment variables:
- SIGNAL_HTTP_URL, SIGNAL_ACCOUNT (required)
- SIGNAL_ALLOWED_USERS, SIGNAL_ALLOWED_GROUPS
- SIGNAL_IGNORE_ATTACHMENTS (default: false)
- SIGNAL_IGNORE_STORIES (default: true)

Includes unit tests covering allowlist logic, envelope parsing,
recipient targeting, SSE deserialization, and edge cases.

* refactor(signal): remove expect|unwrap calls

- Change SignalChannel::new to return Result<Self, ChannelError>
- Replace .expect() on reqwest client build with proper error handling
- Replace .expect() on NonZeroUsize with compile-time const using unsafe new_unchecked
- Propagate errors through test helpers to avoid unwraps in tests

* fix(signal): prevent OOM from chunked response without Content-Length

Use bytes_stream() to check response size during download rather than
buffering entire body first. This closes the OOM vector where a
malicious signal-cli daemon could send unbounded chunked data.

* fix(signal): align is_e164 minimum digits with setup wizard

Both now require 7-15 digits after '+', preventing environment
variable bypass of the stricter onboarding validation.

* refactor(signal): extract from_parts constructor

Extract SignalChannel::from_parts() used by both new() and
sse_listener() to ensure consistent object construction.

* chore: remove redundant unused var

* refactor(signal): rename allowed_users to allow_from and add dm_policy/group_policy

- Rename allowed_users -> allow_from for consistency with other channels
- Rename allowed_groups -> allow_from_groups
- Add dm_policy field: 'open', 'allowlist', or 'pairing' (default: 'pairing')
- Add group_policy field: 'allowlist', 'open', or 'disabled' (default: 'allowlist')
- Add group_allow_from field that inherits from allow_from if empty
- Implement dm_policy and group_policy logic in message processing
- Add environment variable resolution: SIGNAL_ALLOW_FROM, SIGNAL_ALLOW_FROM_GROUPS,
  SIGNAL_DM_POLICY, SIGNAL_GROUP_POLICY, SIGNAL_GROUP_ALLOW_FROM
- Add setup wizard prompts for new policy options
- Note: full pairing flow (PairingStore integration) marked as pending for future PR

* feat(signal): implement DM pairing workflow for unapproved senders

- Add PairingStore integration to check approved senders
- Handle pairing requests for unknown senders with dm_policy=pairing
- Send pairing reply message with approval instructions
- Update FEATURE_PARITY.md to reflect DM pairing support

* chore(ci): fix clippy warnings

FEATURE_PARITY.md

@@ -68,7 +68,7 @@ This document tracks feature parity between IronClaw (Rust implementation) and O
 | WhatsApp | ✅ | ❌ | P1 | Baileys (Web), same-phone mode with echo detection |
 | Telegram | ✅ | ✅ | - | WASM channel(MTProto), DM pairing, caption, /start, bot_username |
 | Discord | ✅ | ❌ | P2 | discord.js, thread parent binding inheritance |
-| Signal | ✅ | ❌ | P2 | signal-cli |
+| Signal | ✅ | ✅ | P2 | signal-cli daemonPC, SSE listener HTTP/JSON-R, user/group allowlists, DM pairing |
 | Slack | ✅ | ✅ | - | WASM tool |
 | iMessage | ✅ | ❌ | P3 | BlueBubbles or Linq recommended |
 | Linq | ✅ | ❌ | P3 | Real iMessage via API, no Mac required |
@@ -540,7 +540,6 @@ This document tracks feature parity between IronClaw (Rust implementation) and O
 
 ### P3 - Lower Priority
 - ❌ Discord channel
-- ❌ Signal channel
 - ❌ Matrix channel
 - ❌ Other messaging platforms
 - ❌ TTS/audio features