fix: don't fail install when running npm install in an example workspace (#687)

[d-turley]

Summary

Fixes #687.

Following an example's README literally — e.g.:

cd examples/basic-host
npm install

fails with a cryptic error:

Error [ERR_MODULE_NOT_FOUND]: Cannot find package 'ts-to-zod' imported from .../scripts/generate-schemas.ts
npm error command sh -c npm run build && husky

Root cause

examples/* are npm workspaces of the repository root. Running npm install
inside an example directory makes npm install only that workspace's dependencies
(it does not install the root package's devDependencies such as ts-to-zod,
bun, tsx, esbuild), yet npm still runs the root package's prepare script
(npm run build && husky). The build immediately runs
generate:schemas → tsx scripts/generate-schemas.ts, which imports the missing
ts-to-zod and crashes. This affects every example, not just basic-host.

Changes

1. Guard the prepare build — move it into scripts/prepare.mjs, which skips
   the SDK build with a clear, actionable message when the build toolchain isn't
   installed (i.e. a workspace-child/partial install) instead of failing with an
   opaque ERR_MODULE_NOT_FOUND. A full install from the repo root still builds
   as before.

   • Registry consumers are unaffected: prepare doesn't run for published
     packages and dist/ ships in the tarball.
   • Git installs still build, because npm installs a git dependency's
     devDependencies before running its prepare.

2. Docs — update the example READMEs' setup instructions to install from the
   repository root (which installs all workspaces and builds the SDK the examples
   import), then run the example from its directory.

Testing

In a clean checkout on Windows:

• cd examples/basic-host && npm install — before: exit 1, ERR_MODULE_NOT_FOUND: ts-to-zod; after: exit 0 with:
  [prepare] Skipping SDK build: missing build tooling (ts-to-zod, bun). ... run npm install from the repository root.
• npm install from the repository root — still runs the full build
  (generate:schemas + sync:snippets + bundle), exits 0, and produces dist/.

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