feat(session): make /handoff actually transfer the session live

Builds on @kshitijk4poor's CLI handoff stub. The original PR's flow
deferred everything to whenever a real user happened to message the
target platform; this rewrites it so the gateway picks up handoffs
immediately and the destination chat just starts working.

State machine on sessions table replaces the boolean flag:
  None -> 'pending' -> 'running' -> ('completed' | 'failed')
plus handoff_error for failure reasons. CLI request_handoff /
get_handoff_state / list_pending_handoffs / claim_handoff /
complete_handoff / fail_handoff helpers wrap the transitions.

CLI side (cli.py): /handoff <platform> validates the platform's home
channel via load_gateway_config, refuses if the agent is mid-turn,
flips the row to 'pending', and poll-blocks (60s) on terminal state.
On 'completed' it prints the /resume hint and exits the CLI like
/quit. On 'failed' or timeout it surfaces the reason and the CLI
session stays intact.

Gateway side (gateway/run.py): new _handoff_watcher background task
scans state.db every 2s, atomically claims pending rows, and runs
_process_handoff for each. _process_handoff:

  1. Resolves the platform's home channel.
  2. Asks the adapter for a fresh thread via the new
     create_handoff_thread(parent_chat_id, name) capability so the
     handed-off conversation gets its own scrollback. Adapters that
     don't support threads (or fail) return None and the watcher
     falls back to the home channel directly.
  3. Constructs a SessionSource keyed as 'thread' when a thread was
     created, 'dm' otherwise, then session_store.switch_session
     re-binds the destination key to the CLI session_id. The full
     role-aware transcript replays via load_transcript on the next
     turn (no flat-text injection into context_prompt).
  4. Forges a synthetic MessageEvent(internal=True) with the handoff
     notice and dispatches through _handle_message; the agent runs
     against the loaded transcript and adapter.send delivers the
     reply.
  5. Marks the row 'completed' on success, 'failed' (+error) on any
     exception.

Adapter capability (gateway/platforms/base.py): create_handoff_thread
default returns None. Three overrides:

  - Telegram (gateway/platforms/telegram.py): wraps _create_dm_topic
    so DM topics (Bot API 9.4+) and forum supergroups both work.
  - Discord (gateway/platforms/discord.py): parent.create_thread on
    text channels with a seed-message + message.create_thread
    fallback for permission edge cases. Skips DMs and other
    non-thread-capable parents.
  - Slack (gateway/platforms/slack.py): posts a seed message and
    returns its ts as the thread anchor — Slack threads are
    message-anchored.

In thread mode, build_session_key keys the destination without
user_id (thread_sessions_per_user defaults to False) so the synthetic
turn and any later real-user message in the thread share the same
session_key — seamless takeover without race.

CommandDef stays cli_only=True (handoff is initiated from the CLI;
gateway exposes /resume for the reverse direction).

Removed the original PR's _handle_message_with_agent handoff hook
(transcript-as-text injection into context_prompt) and the
send_message_tool notification — both replaced by the watcher path.

Tests rewritten around the new state machine: 13/13 pass.
E2E-validated thread + no-thread paths and the failure path against
real worktree imports with mocked adapters.

gateway/platforms/slack.py

@@ -679,6 +679,41 @@ class SlackAdapter(BasePlatformAdapter):
             if lock_acquired and not self._running:
                 self._release_platform_lock()
 
+    async def create_handoff_thread(
+        self,
+        parent_chat_id: str,
+        name: str,
+    ) -> Optional[str]:
+        """Create a Slack thread anchor for a session handoff.
+
+        Slack threads are anchored to a parent message (``thread_ts``), not
+        a channel-level construct. So we post a seed message into the home
+        channel and return its ``ts`` — the watcher uses that as the
+        ``thread_id`` for subsequent sends.
+
+        Returns the seed message ts as a string, or ``None`` on failure.
+        """
+        if not self._app:
+            return None
+        try:
+            client = self._get_client(parent_chat_id)
+            if client is None:
+                return None
+            seed_text = f":thread: Hermes handoff — *{(name or 'session').strip()[:80]}*"
+            result = await client.chat_postMessage(
+                channel=parent_chat_id,
+                text=seed_text,
+            )
+            ts = result.get("ts") if isinstance(result, dict) else getattr(result, "get", lambda _k, _d=None: None)("ts")
+            if ts:
+                return str(ts)
+        except Exception as exc:
+            logger.warning(
+                "[%s] Handoff thread: seed-post failed for channel %s: %s",
+                self.name, parent_chat_id, exc,
+            )
+        return None
+
     async def disconnect(self) -> None:
         """Disconnect from Slack."""
         if self._handler: