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

hermes_cli/main.py

@@ -5004,7 +5004,7 @@ For more help on a command:
     # =========================================================================
     sessions_parser = subparsers.add_parser(
         "sessions",
-        help="Manage session history (list, rename, export, prune, delete)",
+        help="Manage session history (list, rename, export, prune, gc, delete)",
         description="View and manage the SQLite session store"
     )
     sessions_subparsers = sessions_parser.add_subparsers(dest="sessions_action")
@@ -5027,6 +5027,14 @@ For more help on a command:
     sessions_prune.add_argument("--source", help="Only prune sessions from this source")
     sessions_prune.add_argument("--yes", "-y", action="store_true", help="Skip confirmation")
 
+    sessions_gc = sessions_subparsers.add_parser("gc", help="Garbage-collect empty/trivial sessions")
+    sessions_gc.add_argument("--empty-hours", type=int, default=24, help="Delete empty (0-msg) sessions older than N hours (default: 24)")
+    sessions_gc.add_argument("--trivial-days", type=int, default=7, help="Delete trivial (1-5 msg) sessions older than N days (default: 7)")
+    sessions_gc.add_argument("--trivial-max", type=int, default=5, help="Max messages to consider trivial (default: 5)")
+    sessions_gc.add_argument("--source", help="Only GC sessions from this source")
+    sessions_gc.add_argument("--dry-run", action="store_true", help="Show what would be deleted without deleting")
+    sessions_gc.add_argument("--yes", "-y", action="store_true", help="Skip confirmation")
+
     sessions_stats = sessions_subparsers.add_parser("stats", help="Show session store statistics")
 
     sessions_rename = sessions_subparsers.add_parser("rename", help="Set or change a session's title")
@@ -5196,6 +5204,49 @@ For more help on a command:
                 size_mb = os.path.getsize(db_path) / (1024 * 1024)
                 print(f"Database size: {size_mb:.1f} MB")
 
+        elif action == "gc":
+            dry_run = getattr(args, "dry_run", False)
+            if dry_run:
+                counts = db.garbage_collect(
+                    empty_older_than_hours=args.empty_hours,
+                    trivial_max_messages=args.trivial_max,
+                    trivial_older_than_days=args.trivial_days,
+                    source=args.source,
+                    dry_run=True,
+                )
+                print(f"[dry-run] Would delete {counts['total']} session(s):")
+                print(f"  Empty (0 msgs, >{args.empty_hours}h old): {counts['empty']}")
+                print(f"  Trivial (<={args.trivial_max} msgs, >{args.trivial_days}d old): {counts['trivial']}")
+            else:
+                # Preview first
+                preview = db.garbage_collect(
+                    empty_older_than_hours=args.empty_hours,
+                    trivial_max_messages=args.trivial_max,
+                    trivial_older_than_days=args.trivial_days,
+                    source=args.source,
+                    dry_run=True,
+                )
+                if preview["total"] == 0:
+                    print("Nothing to collect.")
+                else:
+                    if not args.yes:
+                        if not _confirm_prompt(
+                            f"Delete {preview['total']} session(s) "
+                            f"({preview['empty']} empty, {preview['trivial']} trivial)? [y/N] "
+                        ):
+                            print("Cancelled.")
+                            return
+                    counts = db.garbage_collect(
+                        empty_older_than_hours=args.empty_hours,
+                        trivial_max_messages=args.trivial_max,
+                        trivial_older_than_days=args.trivial_days,
+                        source=args.source,
+                        dry_run=False,
+                    )
+                    print(f"Collected {counts['total']} session(s):")
+                    print(f"  Empty: {counts['empty']}")
+                    print(f"  Trivial: {counts['trivial']}")
+
         else:
             sessions_parser.print_help()