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

.env.example

@@ -75,6 +75,17 @@ HTTP_HOST=0.0.0.0
 HTTP_PORT=8080
 HTTP_WEBHOOK_SECRET=your-webhook-secret
 
+# Signal Channel (optional, requires signal-cli daemon --http)
+# SIGNAL_HTTP_URL=http://127.0.0.1:8080
+# SIGNAL_ACCOUNT=+1234567890
+# SIGNAL_ALLOW_FROM=+1234567890,uuid:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx  # comma-separated, * for all, empty = deny/require pairing
+# SIGNAL_ALLOW_FROM_GROUPS=                        # comma-separated group IDs, * for all, empty = deny all groups
+# SIGNAL_DM_POLICY=pairing                         # open | allowlist | pairing
+# SIGNAL_GROUP_POLICY=allowlist                    # allowlist | open | disabled
+# SIGNAL_GROUP_ALLOW_FROM=                         # comma-separated, empty = inherit from ALLOW_FROM
+# SIGNAL_IGNORE_ATTACHMENTS=false
+# SIGNAL_IGNORE_STORIES=true
+
 # Agent Settings
 AGENT_NAME=ironclaw
 AGENT_MAX_PARALLEL_JOBS=5