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/session.py

@@ -45,6 +45,8 @@ class SessionSource:
     user_name: Optional[str] = None
     thread_id: Optional[str] = None  # For forum topics, Discord threads, etc.
     chat_topic: Optional[str] = None  # Channel topic/description (Discord, Slack)
+    user_id_alt: Optional[str] = None  # Signal UUID (alternative to phone number)
+    chat_id_alt: Optional[str] = None  # Signal group internal ID
     
     @property
     def description(self) -> str:
@@ -68,7 +70,7 @@ class SessionSource:
         return ", ".join(parts)
     
     def to_dict(self) -> Dict[str, Any]:
-        return {
+        d = {
             "platform": self.platform.value,
             "chat_id": self.chat_id,
             "chat_name": self.chat_name,
@@ -78,6 +80,11 @@ class SessionSource:
             "thread_id": self.thread_id,
             "chat_topic": self.chat_topic,
         }
+        if self.user_id_alt:
+            d["user_id_alt"] = self.user_id_alt
+        if self.chat_id_alt:
+            d["chat_id_alt"] = self.chat_id_alt
+        return d
     
     @classmethod
     def from_dict(cls, data: Dict[str, Any]) -> "SessionSource":
@@ -90,6 +97,8 @@ class SessionSource:
             user_name=data.get("user_name"),
             thread_id=data.get("thread_id"),
             chat_topic=data.get("chat_topic"),
+            user_id_alt=data.get("user_id_alt"),
+            chat_id_alt=data.get("chat_id_alt"),
         )
     
     @classmethod