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_cli/commands.py

@@ -79,6 +79,8 @@ COMMAND_REGISTRY: list[CommandDef] = [
     CommandDef("undo", "Remove the last user/assistant exchange", "Session"),
     CommandDef("title", "Set a title for the current session", "Session",
                args_hint="[name]"),
+    CommandDef("handoff", "Hand off this session to a messaging platform (Telegram, Discord, etc.)", "Session",
+               args_hint="<platform>", cli_only=True),
     CommandDef("branch", "Branch the current session (explore a different path)", "Session",
                aliases=("fork",), args_hint="[name]"),
     CommandDef("compress", "Manually compress conversation context", "Session",