fix(slack): preserve typed command integrity across enrichment paths

Commands typed in Slack could be mangled by every enrichment layer the
adapter applies to normal messages:

- Block Kit / unfurl / attachment-notice / text-file injection could
  prepend or append content around a command, moving the command token
  away from character zero or polluting its arguments. Commands are now
  restored from canonical authored input after all enrichment
  (final is_command_text guard before MessageEvent construction).
- @bot /cmd (typed slash behind a mention) was never classified as a
  command; the mention-strip branch now re-probes for both slash and
  bang forms.
- The Slack Agent-view context label ([Slack app context: ...]) was
  prepended to command events too; now command-exempt.
- Native slash payload arguments were strip()ed, destroying meaningful
  spacing inside/after arguments; only the command delimiter is
  nonsemantic now.
- Slash payload thread identity (thread_ts/message_ts, top-level or
  nested in message/container) is preserved onto SessionSource so
  session-scoped commands hit the same thread session.
- /queue and /steer queued fallbacks now propagate channel_context so a
  command that triggered first-entry thread backfill doesn't lose the
  history when re-queued.

Adapted from PR #66310 to the post-#69320 channel_context design (thread
history already rides MessageEvent.channel_context, never text).

tests/gateway/test_queue_command.py

@@ -175,6 +175,25 @@ async def test_queue_preserves_reply_context():
     assert queued.reply_to_author_name == "alice"
 
 
+@pytest.mark.asyncio
+async def test_queue_preserves_channel_context_backfill():
+    """A queued Slack thread command must retain first-entry history."""
+    runner, adapter = _make_runner(_session_entry())
+    sk = _running(runner)
+    context = "[Thread context]\nAlice: earlier request"
+    event = MessageEvent(
+        text="/queue follow up",
+        source=_make_source(),
+        message_id="q-context",
+        channel_context=context,
+    )
+
+    result = await runner._handle_message(event)
+
+    assert result is not None and "queued" in result.lower()
+    assert adapter._pending_messages[sk].channel_context == context
+
+
 @pytest.mark.asyncio
 async def test_queue_no_text_no_media_returns_usage():
     runner, adapter = _make_runner(_session_entry())