fix(docs): rename apps/docs's start script to match its own convention (#9097)

pnpm docs:start has never worked (both lines landed in 159e299,
2026-08-15): the root delegate calls `pnpm --filter @objectstack/docs
start`, but apps/docs declared `site:start` instead.

Determination: the app's script name is the outlier, not the root
delegate. apps/docs/README.md already documents `pnpm start` (not
`pnpm site:start`) as the local equivalent of `pnpm docs:start`, right
alongside `pnpm dev`/`pnpm build` — which the root's already-working
`docs:dev`/`docs:build` delegates map onto verbatim. No other file in
the repo (workflows, vercel.json, scripts) references `site:start`, so
renaming it is safe. The root package.json needs no change: `docs:start`
was already correctly named.

Fixes #9086

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-project-manager

apps/docs/package.json

@@ -7,7 +7,7 @@
   "scripts": {
     "dev": "next dev",
     "build": "pnpm --filter @objectstack/spec gen:schema && pnpm --filter @objectstack/spec gen:docs && NODE_OPTIONS='--max-old-space-size=4096' next build",
-    "site:start": "next start",
+    "start": "next start",
     "site:lint": "next lint",
     "types:check": "fumadocs-mdx && next typegen && tsc --noEmit",
     "postinstall": "fumadocs-mdx",