fix(discord): strip mention syntax from auto-thread names

Previously a message like `<@&1490963422786093149> help` would spawn a
thread literally named `<@&1490963422786093149> help`, exposing raw
Discord mention markers in the thread list. Only user mentions
(`<@id>`) were being stripped upstream — role mentions (`<@&id>`) and
channel mentions (`<#id>`) leaked through.

Fix: strip all three mention patterns in `_auto_create_thread` before
building the thread name. Collapse runs of whitespace left by the
removal. If the entire content was mention-only, fall back to 'Hermes'
instead of an empty title.

Fixes #6336.

Tests: two new regression guards in test_discord_slash_commands.py
covering mixed-mention content and mention-only content.

gateway/platforms/discord.py

@@ -2407,8 +2407,15 @@ class DiscordAdapter(BasePlatformAdapter):
 
         Returns the created thread object, or ``None`` on failure.
         """
-        # Build a short thread name from the message
+        # Build a short thread name from the message. Strip Discord mention
+        # syntax (users / roles / channels) so thread titles don't end up
+        # showing raw <@id>, <@&id>, or <#id> markers — the ID isn't
+        # meaningful to humans glancing at the thread list (#6336).
         content = (message.content or "").strip()
+        # <@123>, <@!123>, <@&123>, <#123> — collapse to empty; normalize spaces.
+        content = re.sub(r"<@[!&]?\d+>", "", content)
+        content = re.sub(r"<#\d+>", "", content)
+        content = re.sub(r"\s+", " ", content).strip()
         thread_name = content[:80] if content else "Hermes"
         if len(content) > 80:
             thread_name = thread_name[:77] + "..."

tests/gateway/test_discord_slash_commands.py

@@ -414,6 +414,48 @@ async def test_auto_create_thread_uses_message_content_as_name(adapter):
     assert call_kwargs["auto_archive_duration"] == 1440
 
 
+@pytest.mark.asyncio
+async def test_auto_create_thread_strips_mention_syntax_from_name(adapter):
+    """Thread names must not contain raw <@id>, <@&id>, or <#id> markers.
+
+    Regression guard for #6336 — previously a message like
+    ``<@&1490963422786093149> help`` would spawn a thread literally
+    named ``<@&1490963422786093149> help``.
+    """
+    thread = SimpleNamespace(id=999, name="help")
+    message = SimpleNamespace(
+        content="<@&1490963422786093149> <@555> please help <#123>",
+        create_thread=AsyncMock(return_value=thread),
+        channel=SimpleNamespace(send=AsyncMock()),
+        author=SimpleNamespace(display_name="Jezza"),
+    )
+
+    await adapter._auto_create_thread(message)
+
+    name = message.create_thread.await_args[1]["name"]
+    assert "<@" not in name, f"role/user mention leaked: {name!r}"
+    assert "<#" not in name, f"channel mention leaked: {name!r}"
+    assert name == "please help"
+
+
+@pytest.mark.asyncio
+async def test_auto_create_thread_falls_back_to_hermes_when_only_mentions(adapter):
+    """If a message contains only mention syntax, the stripped content is
+    empty — fall back to the 'Hermes' default rather than ''."""
+    thread = SimpleNamespace(id=999, name="Hermes")
+    message = SimpleNamespace(
+        content="<@&1490963422786093149>",
+        create_thread=AsyncMock(return_value=thread),
+        channel=SimpleNamespace(send=AsyncMock()),
+        author=SimpleNamespace(display_name="Jezza"),
+    )
+
+    await adapter._auto_create_thread(message)
+
+    name = message.create_thread.await_args[1]["name"]
+    assert name == "Hermes"
+
+
 @pytest.mark.asyncio
 async def test_auto_create_thread_truncates_long_names(adapter):
     long_text = "a" * 200