feat: add text adventure shell (PLAN.md 3.4)

Zork-style dungeon crawler set in an abandoned data center / medieval dungeon.
11 rooms, 6 items, 3 puzzles (dark room, locked door, maintenance panel),
standard text adventure parser with verb aliases and direction shortcuts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

PLAN.md

@@ -162,7 +162,7 @@ Goal: Add the entertaining shell implementations.
 - "WARNING: milk expires in 2 days"
 - Per-credential shell routing via `shell` field in static credentials
 
-### 3.4 Text Adventure
+### 3.4 Text Adventure ✅
 - Zork-style dungeon crawler
 - "You are in a dimly lit server room."
 - Navigation, items, puzzles