feat: add discord.reactions config option to disable message reactions (#4199)

Adds a 'reactions' key under the discord config section (default: true).
When set to false, the bot no longer adds 👀/✅/❌ reactions to messages
during processing. The config maps to DISCORD_REACTIONS env var following
the same pattern as require_mention and auto_thread.

Files changed:
- hermes_cli/config.py: Add reactions default to DEFAULT_CONFIG
- gateway/config.py: Map discord.reactions to DISCORD_REACTIONS env var
- gateway/platforms/discord.py: Gate on_processing_start/complete hooks
- tests/gateway/test_discord_reactions.py: 3 new tests for config gate

tests/gateway/test_discord_reactions.py

@@ -168,3 +168,67 @@ async def test_reaction_helper_failures_do_not_break_message_flow(adapter):
     await adapter._process_message_background(event, build_session_key(event.source))
 
     adapter.send.assert_awaited_once()
+
+
+@pytest.mark.asyncio
+async def test_reactions_disabled_via_env(adapter, monkeypatch):
+    """When DISCORD_REACTIONS=false, no reactions should be added."""
+    monkeypatch.setenv("DISCORD_REACTIONS", "false")
+
+    raw_message = SimpleNamespace(
+        add_reaction=AsyncMock(),
+        remove_reaction=AsyncMock(),
+    )
+
+    async def handler(_event):
+        await asyncio.sleep(0)
+        return "ack"
+
+    async def hold_typing(_chat_id, interval=2.0, metadata=None):
+        await asyncio.Event().wait()
+
+    adapter.set_message_handler(handler)
+    adapter.send = AsyncMock(return_value=SendResult(success=True, message_id="999"))
+    adapter._keep_typing = hold_typing
+
+    event = _make_event("4", raw_message)
+    await adapter._process_message_background(event, build_session_key(event.source))
+
+    raw_message.add_reaction.assert_not_awaited()
+    raw_message.remove_reaction.assert_not_awaited()
+    # Response should still be sent
+    adapter.send.assert_awaited_once()
+
+
+@pytest.mark.asyncio
+async def test_reactions_disabled_via_env_zero(adapter, monkeypatch):
+    """DISCORD_REACTIONS=0 should also disable reactions."""
+    monkeypatch.setenv("DISCORD_REACTIONS", "0")
+
+    raw_message = SimpleNamespace(
+        add_reaction=AsyncMock(),
+        remove_reaction=AsyncMock(),
+    )
+
+    event = _make_event("5", raw_message)
+    await adapter.on_processing_start(event)
+    await adapter.on_processing_complete(event, success=True)
+
+    raw_message.add_reaction.assert_not_awaited()
+    raw_message.remove_reaction.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_reactions_enabled_by_default(adapter, monkeypatch):
+    """When DISCORD_REACTIONS is unset, reactions should still work (default: true)."""
+    monkeypatch.delenv("DISCORD_REACTIONS", raising=False)
+
+    raw_message = SimpleNamespace(
+        add_reaction=AsyncMock(),
+        remove_reaction=AsyncMock(),
+    )
+
+    event = _make_event("6", raw_message)
+    await adapter.on_processing_start(event)
+
+    raw_message.add_reaction.assert_awaited_once_with("👀")