feat: add Signal messenger gateway platform (#405)

Complete Signal adapter using signal-cli daemon HTTP API.
Based on PR #268 by ibhagwan, rebuilt on current main with bug fixes.

Architecture:
- SSE streaming for inbound messages with exponential backoff (2s→60s)
- JSON-RPC 2.0 for outbound (send, typing, attachments, contacts)
- Health monitor detects stale SSE connections (120s threshold)
- Phone number redaction in all logs and global redact.py

Features:
- DM and group message support with separate access policies
- DM policies: pairing (default), allowlist, open
- Group policies: disabled (default), allowlist, open
- Attachment download with magic-byte type detection
- Typing indicators (8s refresh interval)
- 100MB attachment size limit, 8000 char message limit
- E.164 phone + UUID allowlist support

Integration:
- Platform.SIGNAL enum in gateway/config.py
- Signal in _is_user_authorized() allowlist maps (gateway/run.py)
- Adapter factory in _create_adapter() (gateway/run.py)
- user_id_alt/chat_id_alt fields in SessionSource for UUIDs
- send_message tool support via httpx JSON-RPC (not aiohttp)
- Interactive setup wizard in 'hermes gateway setup'
- Connectivity testing during setup (pings /api/v1/check)
- signal-cli detection and install guidance

Bug fixes from PR #268:
- Timestamp reads from envelope_data (not outer wrapper)
- Uses httpx consistently (not aiohttp in send_message tool)
- SIGNAL_DEBUG scoped to signal logger (not root)
- extract_images regex NOT modified (preserves group numbering)
- pairing.py NOT modified (no cross-platform side effects)
- No dual authorization (adapter defers to run.py for user auth)
- Wildcard uses set membership ('*' in set, not list equality)
- .zip default for PK magic bytes (not .docx)

No new Python dependencies — uses httpx (already core).
External requirement: signal-cli daemon (user-installed).

Tests: 30 new tests covering config, init, helpers, session source,
phone redaction, authorization, and send_message integration.

Co-authored-by: ibhagwan <ibhagwan@users.noreply.github.com>

gateway/run.py

@@ -591,6 +591,13 @@ class GatewayRunner:
                 return None
             return SlackAdapter(config)
 
+        elif platform == Platform.SIGNAL:
+            from gateway.platforms.signal import SignalAdapter, check_signal_requirements
+            if not check_signal_requirements():
+                logger.warning("Signal: SIGNAL_HTTP_URL or SIGNAL_ACCOUNT not configured")
+                return None
+            return SignalAdapter(config)
+
         elif platform == Platform.HOMEASSISTANT:
             from gateway.platforms.homeassistant import HomeAssistantAdapter, check_ha_requirements
             if not check_ha_requirements():
@@ -626,12 +633,14 @@ class GatewayRunner:
             Platform.DISCORD: "DISCORD_ALLOWED_USERS",
             Platform.WHATSAPP: "WHATSAPP_ALLOWED_USERS",
             Platform.SLACK: "SLACK_ALLOWED_USERS",
+            Platform.SIGNAL: "SIGNAL_ALLOWED_USERS",
         }
         platform_allow_all_map = {
             Platform.TELEGRAM: "TELEGRAM_ALLOW_ALL_USERS",
             Platform.DISCORD: "DISCORD_ALLOW_ALL_USERS",
             Platform.WHATSAPP: "WHATSAPP_ALLOW_ALL_USERS",
             Platform.SLACK: "SLACK_ALLOW_ALL_USERS",
+            Platform.SIGNAL: "SIGNAL_ALLOW_ALL_USERS",
         }
 
         # Per-platform allow-all flag (e.g., DISCORD_ALLOW_ALL_USERS=true)