fix(discord): persist thread participation across gateway restarts

_bot_participated_threads was an in-memory set — lost on every restart.
After restart, the bot forgot which threads it was active in, requiring
fresh @mentions and potentially creating duplicate threads instead of
continuing existing conversations.

Changes:
- Persist thread IDs to ~/.hermes/discord_threads.json
- Load on adapter init, save on every new thread participation
- _track_thread() replaces direct .add() calls for atomic persist
- Cap at 500 tracked threads to prevent unbounded growth
- /thread slash command also tracks participation
- 7 new tests covering persistence, restart survival, corruption
  recovery, cap enforcement

tests/gateway/test_discord_thread_persistence.py

@@ -0,0 +1,83 @@
+"""Tests for Discord thread participation persistence.
+
+Verifies that _bot_participated_threads survives adapter restarts by
+being persisted to ~/.hermes/discord_threads.json.
+"""
+
+import json
+import os
+from unittest.mock import patch
+
+import pytest
+
+
+class TestDiscordThreadPersistence:
+    """Thread IDs are saved to disk and reloaded on init."""
+
+    def _make_adapter(self, tmp_path):
+        """Build a minimal DiscordAdapter with HERMES_HOME pointed at tmp_path."""
+        from gateway.config import PlatformConfig
+        from gateway.platforms.discord import DiscordAdapter
+
+        config = PlatformConfig(enabled=True, token="test-token")
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            return DiscordAdapter(config=config)
+
+    def test_starts_empty_when_no_state_file(self, tmp_path):
+        adapter = self._make_adapter(tmp_path)
+        assert adapter._bot_participated_threads == set()
+
+    def test_track_thread_persists_to_disk(self, tmp_path):
+        adapter = self._make_adapter(tmp_path)
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            adapter._track_thread("111")
+            adapter._track_thread("222")
+
+        state_file = tmp_path / "discord_threads.json"
+        assert state_file.exists()
+        saved = json.loads(state_file.read_text())
+        assert set(saved) == {"111", "222"}
+
+    def test_threads_survive_restart(self, tmp_path):
+        """Threads tracked by one adapter instance are visible to the next."""
+        adapter1 = self._make_adapter(tmp_path)
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            adapter1._track_thread("aaa")
+            adapter1._track_thread("bbb")
+
+        adapter2 = self._make_adapter(tmp_path)
+        assert "aaa" in adapter2._bot_participated_threads
+        assert "bbb" in adapter2._bot_participated_threads
+
+    def test_duplicate_track_does_not_double_save(self, tmp_path):
+        adapter = self._make_adapter(tmp_path)
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            adapter._track_thread("111")
+            adapter._track_thread("111")  # no-op
+
+        saved = json.loads((tmp_path / "discord_threads.json").read_text())
+        assert saved.count("111") == 1
+
+    def test_caps_at_max_tracked_threads(self, tmp_path):
+        adapter = self._make_adapter(tmp_path)
+        adapter._MAX_TRACKED_THREADS = 5
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            for i in range(10):
+                adapter._track_thread(str(i))
+
+        assert len(adapter._bot_participated_threads) == 5
+
+    def test_corrupted_state_file_falls_back_to_empty(self, tmp_path):
+        state_file = tmp_path / "discord_threads.json"
+        state_file.write_text("not valid json{{{")
+        adapter = self._make_adapter(tmp_path)
+        assert adapter._bot_participated_threads == set()
+
+    def test_missing_hermes_home_does_not_crash(self, tmp_path):
+        """Load/save tolerate missing directories."""
+        fake_home = tmp_path / "nonexistent" / "deep"
+        with patch.dict(os.environ, {"HERMES_HOME": str(fake_home)}):
+            from gateway.platforms.discord import DiscordAdapter
+            # _load should return empty set, not crash
+            threads = DiscordAdapter._load_participated_threads()
+            assert threads == set()