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/config.py

@@ -26,6 +26,7 @@ class Platform(Enum):
     DISCORD = "discord"
     WHATSAPP = "whatsapp"
     SLACK = "slack"
+    SIGNAL = "signal"
     HOMEASSISTANT = "homeassistant"
 
 
@@ -155,7 +156,16 @@ class GatewayConfig:
         """Return list of platforms that are enabled and configured."""
         connected = []
         for platform, config in self.platforms.items():
-            if config.enabled and (config.token or config.api_key):
+            if not config.enabled:
+                continue
+            # Platforms that use token/api_key auth
+            if config.token or config.api_key:
+                connected.append(platform)
+            # WhatsApp uses enabled flag only (bridge handles auth)
+            elif platform == Platform.WHATSAPP:
+                connected.append(platform)
+            # Signal uses extra dict for config (http_url + account)
+            elif platform == Platform.SIGNAL and config.extra.get("http_url"):
                 connected.append(platform)
         return connected
     
@@ -379,6 +389,28 @@ def _apply_env_overrides(config: GatewayConfig) -> None:
                 name=os.getenv("SLACK_HOME_CHANNEL_NAME", ""),
             )
     
+    # Signal
+    signal_url = os.getenv("SIGNAL_HTTP_URL")
+    signal_account = os.getenv("SIGNAL_ACCOUNT")
+    if signal_url and signal_account:
+        if Platform.SIGNAL not in config.platforms:
+            config.platforms[Platform.SIGNAL] = PlatformConfig()
+        config.platforms[Platform.SIGNAL].enabled = True
+        config.platforms[Platform.SIGNAL].extra.update({
+            "http_url": signal_url,
+            "account": signal_account,
+            "dm_policy": os.getenv("SIGNAL_DM_POLICY", "pairing"),
+            "group_policy": os.getenv("SIGNAL_GROUP_POLICY", "disabled"),
+            "ignore_stories": os.getenv("SIGNAL_IGNORE_STORIES", "true").lower() in ("true", "1", "yes"),
+        })
+        signal_home = os.getenv("SIGNAL_HOME_CHANNEL")
+        if signal_home:
+            config.platforms[Platform.SIGNAL].home_channel = HomeChannel(
+                platform=Platform.SIGNAL,
+                chat_id=signal_home,
+                name=os.getenv("SIGNAL_HOME_CHANNEL_NAME", "Home"),
+            )
+
     # Home Assistant
     hass_token = os.getenv("HASS_TOKEN")
     if hass_token: