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/overview.md

@@ -0,0 +1,25 @@
+# Docs
+
+The documentation is split into the living specs, which describe how the
+system works today, and the status/plans, which track development iterations
+and go stale as work lands.
+
+## Specs — how the system works
+
+| Doc | Covers |
+| --- | --- |
+| [`architecture.md`](./architecture.md) | System architecture and the package dependency graph |
+| [`decisions.md`](./decisions.md) | Key design decisions and the rationale behind them |
+| [`bgm/format.md`](./bgm/format.md) | The board game manifest (bgm) format spec |
+| [`bgm/engine.md`](./bgm/engine.md) | The bgm message layer: queue, triggers, orchestrators |
+| [`bgm/commands.md`](./bgm/commands.md) | bgm command execution: lifecycle, run contexts, tap interaction |
+| [`bgm/tabletop.md`](./bgm/tabletop.md) | The r3f tabletop component library |
+
+## Status & plans (dev logs)
+
+| Doc | Covers |
+| --- | --- |
+| [`status/implementation-plan.md`](./status/implementation-plan.md) | Original build plan |
+| [`status/bgm-loader.md`](./status/bgm-loader.md) | bgm loader — what's built, works, missing |
+| [`status/bgm-tabletop.md`](./status/bgm-tabletop.md) | bgm tabletop — implementation plan / status |
+| [`status/full-setup-view.md`](./status/full-setup-view.md) | Full-setup view plan |