feat: session garbage collection (#315)

Add garbage_collect() method to SessionDB that cleans up empty and
trivial sessions based on age:
- Empty sessions (0 messages) older than 24h
- Trivial sessions (1-5 messages) older than 7 days
- Sessions with >5 messages kept indefinitely

Add `hermes sessions gc` CLI command with:
- --empty-hours (default: 24)
- --trivial-days (default: 7)
- --trivial-max (default: 5)
- --source filter
- --dry-run preview mode
- --yes skip confirmation

The dry-run flow: preview what would be deleted, ask for confirmation,
then execute. Handles child session FK constraints properly.

7 tests covering: empty/trivial deletion, active session protection,
substantial session preservation, dry-run, source filtering, and child
session handling.

Closes #315

tests/test_hermes_state.py

@@ -665,6 +665,127 @@ class TestPruneSessions:
 
 
 # =========================================================================
+# =========================================================================
+# Garbage Collect
+# =========================================================================
+
+class TestGarbageCollect:
+    def test_gc_deletes_empty_old_sessions(self, db):
+        """Empty sessions (0 messages) older than 24h should be deleted."""
+        db.create_session(session_id="empty_old", source="cli")
+        db.end_session("empty_old", end_reason="done")
+        db._conn.execute(
+            "UPDATE sessions SET started_at = ? WHERE id = ?",
+            (time.time() - 48 * 3600, "empty_old"),  # 48 hours ago
+        )
+        db._conn.commit()
+
+        # Recent empty session should be kept
+        db.create_session(session_id="empty_new", source="cli")
+        db.end_session("empty_new", end_reason="done")
+
+        result = db.garbage_collect()
+        assert result["empty"] == 1
+        assert result["trivial"] == 0
+        assert result["total"] == 1
+        assert db.get_session("empty_old") is None
+        assert db.get_session("empty_new") is not None
+
+    def test_gc_deletes_trivial_old_sessions(self, db):
+        """Sessions with 1-5 messages older than 7 days should be deleted."""
+        db.create_session(session_id="trivial_old", source="cli")
+        for i in range(3):
+            db.append_message("trivial_old", role="user", content=f"msg {i}")
+        db.end_session("trivial_old", end_reason="done")
+        db._conn.execute(
+            "UPDATE sessions SET started_at = ? WHERE id = ?",
+            (time.time() - 10 * 86400, "trivial_old"),  # 10 days ago
+        )
+        db._conn.commit()
+
+        result = db.garbage_collect()
+        assert result["trivial"] == 1
+        assert result["total"] == 1
+        assert db.get_session("trivial_old") is None
+
+    def test_gc_keeps_active_sessions(self, db):
+        """Active (not ended) sessions should never be deleted."""
+        db.create_session(session_id="active_old", source="cli")
+        # Backdate but don't end
+        db._conn.execute(
+            "UPDATE sessions SET started_at = ? WHERE id = ?",
+            (time.time() - 48 * 3600, "active_old"),
+        )
+        db._conn.commit()
+
+        result = db.garbage_collect()
+        assert result["total"] == 0
+        assert db.get_session("active_old") is not None
+
+    def test_gc_keeps_substantial_sessions(self, db):
+        """Sessions with >5 messages should never be deleted."""
+        db.create_session(session_id="big_old", source="cli")
+        for i in range(10):
+            db.append_message("big_old", role="user", content=f"msg {i}")
+        db.end_session("big_old", end_reason="done")
+        db._conn.execute(
+            "UPDATE sessions SET started_at = ? WHERE id = ?",
+            (time.time() - 365 * 86400, "big_old"),  # 1 year ago
+        )
+        db._conn.commit()
+
+        result = db.garbage_collect()
+        assert result["total"] == 0
+        assert db.get_session("big_old") is not None
+
+    def test_gc_dry_run_does_not_delete(self, db):
+        """dry_run=True should return counts but not delete anything."""
+        db.create_session(session_id="empty_old", source="cli")
+        db.end_session("empty_old", end_reason="done")
+        db._conn.execute(
+            "UPDATE sessions SET started_at = ? WHERE id = ?",
+            (time.time() - 48 * 3600, "empty_old"),
+        )
+        db._conn.commit()
+
+        result = db.garbage_collect(dry_run=True)
+        assert result["total"] == 1
+        assert db.get_session("empty_old") is not None  # Still exists
+
+    def test_gc_with_source_filter(self, db):
+        """--source should only GC sessions from that source."""
+        for sid, src in [("old_cli", "cli"), ("old_tg", "telegram")]:
+            db.create_session(session_id=sid, source=src)
+            db.end_session(sid, end_reason="done")
+            db._conn.execute(
+                "UPDATE sessions SET started_at = ? WHERE id = ?",
+                (time.time() - 48 * 3600, sid),
+            )
+        db._conn.commit()
+
+        result = db.garbage_collect(source="cli")
+        assert result["total"] == 1
+        assert db.get_session("old_cli") is None
+        assert db.get_session("old_tg") is not None
+
+    def test_gc_handles_child_sessions(self, db):
+        """Child sessions should be deleted when parent is GC'd."""
+        db.create_session(session_id="parent_old", source="cli")
+        db.end_session("parent_old", end_reason="done")
+        db._conn.execute(
+            "UPDATE sessions SET started_at = ? WHERE id = ?",
+            (time.time() - 48 * 3600, "parent_old"),
+        )
+        # Create child session
+        db.create_session(session_id="child", source="cli", parent_session_id="parent_old")
+        db.end_session("child", end_reason="done")
+        db._conn.commit()
+
+        result = db.garbage_collect()
+        assert result["total"] == 1
+        assert db.get_session("parent_old") is None
+        assert db.get_session("child") is None
+
 # Schema and WAL mode
 # =========================================================================