feat: add Cisco IOS shell with mode state machine and abbreviation matching (PLAN.md 3.2)

Implements a Cisco IOS CLI emulator with four modes (user exec, privileged exec,
global config, interface config), Cisco-style command abbreviation (e.g. sh run,
conf t), enable password flow, and realistic show command output including
running-config, interfaces, IP routes, and VLANs.

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

oubliette.toml.example

@@ -29,6 +29,11 @@ password = "admin"
 # password = "banking"
 # shell = "banking"
 
+# [[auth.static_credentials]]
+# username = "admin"
+# password = "cisco"
+# shell = "cisco"
+
 [storage]
 db_path = "oubliette.db"
 retention_days = 90
@@ -53,6 +58,12 @@ hostname = "ubuntu-server"
 # [shell.adventure]
 # dungeon_name = "THE OUBLIETTE"
 
+# [shell.cisco]
+# hostname = "Router"
+# model = "C2960"
+# ios_version = "15.0(2)SE11"
+# enable_password = ""  # empty = accept after 1 failed attempt
+
 # [detection]
 # enabled = true
 # threshold = 0.6          # 0.0–1.0, sessions above this trigger notifications