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/flow_runner.rb

@@ -72,6 +72,7 @@ module BBS
       when :output        then run_output(step)
       when :tui           then run_tui(step)
       when :app           then run_app(step)
+      when :os            then run_os(step)
       end
     end
 
@@ -359,6 +360,13 @@ module BBS
       :halt
     end
 
+    def run_os(step)
+      FakeOS::Runner.new(@session, step[:definition], context: @ctx).run
+      nil
+    rescue IOError, Errno::EPIPE, Errno::ECONNRESET
+      :halt
+    end
+
     # ── helpers ────────────────────────────────────────────────────────────────
 
     def strip_md(text)