feat(session): add /handoff command for cross-platform session transfer

Adds /handoff <platform> CLI command that queues the current session for
resume on the configured home channel of any messaging platform.

CLI side:
- /handoff telegram — marks session in shared DB, sends summary to
  the Telegram home channel via send_message
- /handoff discord — same for Discord
- Supports telegram, discord, slack, whatsapp, signal, matrix

Gateway side:
- On new session creation, checks for pending handoffs for the
  incoming message's platform
- If found, loads the CLI session's full conversation history and
  injects it into the context prompt as a handoff transcript
- Agent continues the conversation seamlessly

Files:
- hermes_state.py: handoff_pending, handoff_platform columns + helpers
- cli.py: _handle_handoff_command dispatch + handler
- hermes_cli/commands.py: CommandDef entry
- gateway/run.py: handoff detection in _handle_message_with_agent
- tests/hermes_cli/test_session_handoff.py: 8 tests

hermes_state.py

@@ -215,6 +215,8 @@ CREATE TABLE IF NOT EXISTS sessions (
     pricing_version TEXT,
     title TEXT,
     api_call_count INTEGER DEFAULT 0,
+    handoff_pending INTEGER DEFAULT 0,
+    handoff_platform TEXT,
     FOREIGN KEY (parent_session_id) REFERENCES sessions(id)
 );
 
@@ -2861,3 +2863,46 @@ class SessionDB:
 
         return result
 
+    # ── Handoff (cross-platform session transfer) ──────────────────────────
+
+    def set_handoff_pending(self, session_id: str, platform: str) -> bool:
+        """Mark a session as pending handoff to the given platform.
+
+        Returns True if the session was found and updated.
+        """
+        def _do(conn):
+            cur = conn.execute(
+                "UPDATE sessions SET handoff_pending = 1, handoff_platform = ? "
+                "WHERE id = ? AND handoff_pending = 0",
+                (platform, session_id),
+            )
+            return cur.rowcount > 0
+        return self._execute_write(_do)
+
+    def find_pending_handoff(self, platform: str) -> Optional[Dict[str, Any]]:
+        """Find the most recent session pending handoff for a platform.
+
+        Returns the session dict or None.
+        """
+        try:
+            cur = self._conn.execute(
+                "SELECT * FROM sessions "
+                "WHERE handoff_pending = 1 AND handoff_platform = ? "
+                "ORDER BY started_at DESC LIMIT 1",
+                (platform,),
+            )
+            row = cur.fetchone()
+            return dict(row) if row else None
+        except Exception:
+            return None
+
+    def clear_handoff_pending(self, session_id: str) -> None:
+        """Clear the handoff_pending flag on a session."""
+        def _do(conn):
+            conn.execute(
+                "UPDATE sessions SET handoff_pending = 0, handoff_platform = NULL "
+                "WHERE id = ?",
+                (session_id,),
+            )
+        self._execute_write(_do)
+