docs: reorganize docs into bgm and status folders

Group the bgm spec cluster under docs/bgm and move dev logs and plans under docs/status, add an overview index, and update cross-references in the docs, README, and source comments.

docs/decisions.md

@@ -2,7 +2,7 @@
 
 > **Scope:** The rationale behind key design decisions. For the system's
 > architecture, see [`architecture.md`](./architecture.md). For the concrete
-> build plan, see [`implementation-plan.md`](./implementation-plan.md).
+> build plan, see [`status/implementation-plan.md`](./status/implementation-plan.md).
 >
 > Each entry records the decision, the context, and the alternatives considered.
 > New entries are appended; existing entries are updated only to correct facts,
@@ -275,7 +275,7 @@ group starts (e.g. a `camera` group so a second focus cancels the first).
 **Context:** The user wants to script interaction sequences — focus, caption,
 title, highlight, tap-to-advance, move, camera away. The state store and
 render layer already exist; what's missing is a way to drive them over time
-and react to input. Design: [`bgm-commands.md`](./bgm-commands.md).
+and react to input. Design: [`bgm/commands.md`](./bgm/commands.md).
 
 **Alternatives considered:** A single monolithic script interpreter. Rejected
 — commands as self-contained async units are testable in isolation and let