feat: implement channel directory and message mirroring for cross-platform communication

- Introduced a new channel directory to cache reachable channels/contacts for messaging platforms, enhancing the send_message tool's ability to resolve human-friendly names to numeric IDs.
- Added functionality to mirror sent messages into the target's session transcript, providing context for cross-platform message delivery.
- Updated the send_message tool to support listing available targets and improved error handling for channel resolution.
- Enhanced the gateway to build and refresh the channel directory during startup and at regular intervals, ensuring up-to-date channel information.

gateway/mirror.py

@@ -0,0 +1,123 @@
+"""
+Session mirroring for cross-platform message delivery.
+
+When a message is sent to a platform (via send_message or cron delivery),
+this module appends a "delivery-mirror" record to the target session's
+transcript so the receiving-side agent has context about what was sent.
+
+Standalone -- works from CLI, cron, and gateway contexts without needing
+the full SessionStore machinery.
+"""
+
+import json
+import logging
+from datetime import datetime
+from pathlib import Path
+from typing import Optional
+
+logger = logging.getLogger(__name__)
+
+_SESSIONS_DIR = Path.home() / ".hermes" / "sessions"
+_SESSIONS_INDEX = _SESSIONS_DIR / "sessions.json"
+
+
+def mirror_to_session(
+    platform: str,
+    chat_id: str,
+    message_text: str,
+    source_label: str = "cli",
+) -> bool:
+    """
+    Append a delivery-mirror message to the target session's transcript.
+
+    Finds the gateway session that matches the given platform + chat_id,
+    then writes a mirror entry to both the JSONL transcript and SQLite DB.
+
+    Returns True if mirrored successfully, False if no matching session or error.
+    All errors are caught -- this is never fatal.
+    """
+    try:
+        session_id = _find_session_id(platform, str(chat_id))
+        if not session_id:
+            logger.debug("Mirror: no session found for %s:%s", platform, chat_id)
+            return False
+
+        mirror_msg = {
+            "role": "assistant",
+            "content": message_text,
+            "timestamp": datetime.now().isoformat(),
+            "mirror": True,
+            "mirror_source": source_label,
+        }
+
+        _append_to_jsonl(session_id, mirror_msg)
+        _append_to_sqlite(session_id, mirror_msg)
+
+        logger.debug("Mirror: wrote to session %s (from %s)", session_id, source_label)
+        return True
+
+    except Exception as e:
+        logger.debug("Mirror failed for %s:%s: %s", platform, chat_id, e)
+        return False
+
+
+def _find_session_id(platform: str, chat_id: str) -> Optional[str]:
+    """
+    Find the active session_id for a platform + chat_id pair.
+
+    Scans sessions.json entries and matches where origin.chat_id == chat_id
+    on the right platform.  DM session keys don't embed the chat_id
+    (e.g. "agent:main:telegram:dm"), so we check the origin dict.
+    """
+    if not _SESSIONS_INDEX.exists():
+        return None
+
+    try:
+        with open(_SESSIONS_INDEX) as f:
+            data = json.load(f)
+    except Exception:
+        return None
+
+    platform_lower = platform.lower()
+    best_match = None
+    best_updated = ""
+
+    for _key, entry in data.items():
+        origin = entry.get("origin") or {}
+        entry_platform = (origin.get("platform") or entry.get("platform", "")).lower()
+
+        if entry_platform != platform_lower:
+            continue
+
+        origin_chat_id = str(origin.get("chat_id", ""))
+        if origin_chat_id == str(chat_id):
+            updated = entry.get("updated_at", "")
+            if updated > best_updated:
+                best_updated = updated
+                best_match = entry.get("session_id")
+
+    return best_match
+
+
+def _append_to_jsonl(session_id: str, message: dict) -> None:
+    """Append a message to the JSONL transcript file."""
+    transcript_path = _SESSIONS_DIR / f"{session_id}.jsonl"
+    try:
+        with open(transcript_path, "a") as f:
+            f.write(json.dumps(message, ensure_ascii=False) + "\n")
+    except Exception as e:
+        logger.debug("Mirror JSONL write failed: %s", e)
+
+
+def _append_to_sqlite(session_id: str, message: dict) -> None:
+    """Append a message to the SQLite session database."""
+    try:
+        from hermes_state import SessionDB
+        db = SessionDB()
+        db.append_message(
+            session_id=session_id,
+            role=message.get("role", "assistant"),
+            content=message.get("content"),
+        )
+    except Exception as e:
+        logger.debug("Mirror SQLite write failed: %s", e)