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>

oubliette.toml.example

@@ -49,6 +49,9 @@ hostname = "ubuntu-server"
 # terminal_id = "SB-0001"   # random if not set
 # region = "NORTHEAST"
 
+# [shell.adventure]
+# dungeon_name = "THE OUBLIETTE"
+
 # [detection]
 # enabled = true
 # threshold = 0.6          # 0.0–1.0, sessions above this trigger notifications