docs: browser runtime design (craft-libs, workers, #1221) (#1258)

A design note for SolidCraft in the browser: tiered storage, a SharedWorker job broker, GPU dispatch and in-browser agent control through the same engine commands, with the generic pieces in craft-libs. Linked from the MCP and web getting-started docs.

docs/mcp.md

@@ -10,6 +10,11 @@ ordinary `execute` tool: `render.preflight`, `render.start`, `render.status`,
 `render.cancel` and `render.export`. Keep the complete returned job ID; image bytes remain
 in the session until explicit PNG export. See [render jobs](render-jobs.md).
 
+For how the same command surface should work in the browser (workers, tiered storage, in-tab
+agents), see [web-runtime.md](web-runtime.md). For sandboxed plugins and parametric timeline
+features that also use `run(command, args)`, see [plugins.md](plugins.md). Desktop MCP in this
+document remains the reference implementation today.
+
 ## Start it
 
 ```sh