fix(gateway): slash commands never interrupt a running agent (#12334)

Any recognized slash command now bypasses the Level-1 active-session
guard instead of queueing + interrupting. A mid-run /model (or
/reasoning, /voice, /insights, /title, /resume, /retry, /undo,
/compress, /usage, /provider, /reload-mcp, /sethome, /reset) used to
interrupt the agent AND get silently discarded by the slash-command
safety net — zero-char response, dropped tool calls.

Root cause:
- Discord registers 41 native slash commands via tree.command().
- Only 14 were in ACTIVE_SESSION_BYPASS_COMMANDS.
- The other ~15 user-facing ones fell through base.py:handle_message
  to the busy-session handler, which calls running_agent.interrupt()
  AND queues the text.
- After the aborted run, gateway/run.py:9912 correctly identifies the
  queued text as a slash command and discards it — but the damage
  (interrupt + zero-char response) already happened.

Fix:
- should_bypass_active_session() now returns True for any resolvable
  slash command. ACTIVE_SESSION_BYPASS_COMMANDS stays as the subset
  with dedicated Level-2 handlers (documentation + tests).
- gateway/run.py adds a catch-all after the dedicated handlers that
  returns a user-visible "agent busy — wait or /stop first" response
  for any other resolvable command.
- Unknown text / file-path-like messages are unchanged — they still
  queue.

Also:
- gateway/platforms/discord.py logs the invoker identity on every
  slash command (user id + name + channel + guild) so future
  ghost-command reports can be triaged without guessing.

Tests:
- 15 new parametrized cases in test_command_bypass_active_session.py
  cover every previously-broken Discord slash command.
- Existing tests for /stop, /new, /approve, /deny, /help, /status,
  /agents, /background, /steer, /update, /queue still pass.
- test_steer.py's ACTIVE_SESSION_BYPASS_COMMANDS check still passes.

Fixes #5057. Related: #6252, #10370, #4665.

gateway/platforms/discord.py

@@ -1933,6 +1933,24 @@ class DiscordAdapter(BasePlatformAdapter):
         the "thinking..." indicator is replaced with that text; otherwise it
         is deleted so the channel isn't cluttered.
         """
+        # Log the invoker so ghost-command reports can be triaged.  Discord
+        # native slash invocations are always user-initiated (no bot can fire
+        # them), but mobile autocomplete / keyboard shortcuts / other users
+        # in the same channel are easy to miss in post-mortems.
+        try:
+            _user = interaction.user
+            _chan_id = getattr(interaction.channel, "id", None) or getattr(interaction, "channel_id", None)
+            logger.info(
+                "[Discord] slash '%s' invoked by user=%s id=%s channel=%s guild=%s",
+                command_text,
+                getattr(_user, "name", "?"),
+                getattr(_user, "id", "?"),
+                _chan_id,
+                getattr(interaction, "guild_id", None),
+            )
+        except Exception:
+            pass  # logging must never block command dispatch
+
         await interaction.response.defer(ephemeral=True)
         event = self._build_slash_event(interaction, command_text)
         await self.handle_message(event)