docs: design command execution for tabletop scripts

Add bgm-commands.md covering async commands with ok/cancel/error
results, per-invocation run contexts, fire-and-forget vs self-managed
waiting, and tap interaction with part-local trigger points. Link it
from the tabletop design and plan docs, and record the decisions in
decisions.md.

docs/bgm-tabletop-plan.md

@@ -172,6 +172,14 @@ consumers share them (see Open decisions).
 - `pnpm --filter @tts/web build` — the part inspection route renders `PartView`
   from the library (work item 2), proving it end-to-end.
 
+## Commands (not yet implemented)
+
+Scripted interaction is designed in [`bgm-commands.md`](./bgm-commands.md):
+async commands with `ok`/`cancel`/`error` results, per-invocation run
+contexts, fire-and-forget vs self-managed waiting, and tap interaction with
+part-local trigger points. Implementation order: types + run-context manager,
+tap detection, then the first commands (`wait: tap`, `focus`).
+
 ## Open decisions (defaults in bold)
 
 - **Where the trace/sprite helpers live** — **lift into `@tts/mesh`** (shared