feat: add tetris shell (Tetris game TUI)

Full-screen Tetris game using Bubbletea with title screen, ghost piece,
lock delay, NES-style scoring, configurable difficulty (easy/normal/hard),
and honeypot event logging. Bumps version to 0.17.0.

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

oubliette.toml.example

@@ -39,6 +39,11 @@ password = "admin"
 # password = "roomba"
 # shell = "roomba"
 
+# [[auth.static_credentials]]
+# username = "player"
+# password = "tetris"
+# shell = "tetris"
+
 [storage]
 db_path = "oubliette.db"
 retention_days = 90
@@ -83,6 +88,9 @@ hostname = "ubuntu-server"
 # [shell.roomba]
 # No configuration options currently.
 
+# [shell.tetris]
+# difficulty = "normal"  # "easy" (slower start), "normal" (standard), "hard" (start at level 5)
+
 # [detection]
 # enabled = true
 # threshold = 0.6          # 0.0–1.0, sessions above this trigger notifications