- builtin-registry.ts held a 2341-line array of every command spec with its
handler inlined, plus the autocomplete builders and the TUI dispatcher.
- Specs moved verbatim into modes, collaboration, session, lifecycle,
marketplace and control modules; completions moved to builtin-completions.
- builtin-registry.ts is now a 174-line composition/dispatch module and keeps
all 16 exports. Command order is unchanged, which matters because it drives
autocomplete ordering and BUILTIN_SLASH_COMMAND_DEFS.
