Add FakeOS: configurable character-mode OS simulator

BBS::FakeOS is a generic, teletype-style engine for simulating retro
operating systems: a scripted boot sequence (BIOS POST, memory test,
banners, typewriter output) followed by an interactive command shell.

Each simulated OS (MS-DOS, CP/M, AmigaDOS CLI, ...) is just a
FakeOS.define { ... } config — prompt, colors, boot block, command table
and an on_unknown fallback — so new OSes are added as configuration, not
code. on_unknown is the natural hook for a conversational/LLM-backed
shell. Exposed as a Flow step (`os <definition>`) alongside tui/app.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/bbs.rb

@@ -6,6 +6,7 @@ require_relative 'bbs/telnet'
 require_relative 'bbs/banner'
 require_relative 'bbs/flow'
 require_relative 'bbs/flow_runner'
+require_relative 'bbs/fake_os'
 require_relative 'bbs/renderer'
 require_relative 'bbs/store'
 require_relative 'bbs/frame_buffer'