fix(dsh-runtime): add prepare script so git installs build dist

[VedantMadane]

Fixes #7471

Why

Installing @open-design/dsh-runtime as a git dependency (github:nexu-io/open-design#path:packages/dsh-runtime) leaves main / exports pointing at dist/*.js that were never built. pnpm runs a git dependency's prepare hook at install time, but this package only declared build — so install completed with no dist/, and DSH Desktop then hard-failed the whole plugin tree (atomic loader) with ERR_MODULE_NOT_FOUND for dist/startup.js and dist/index.js.

Hit this while wiring the package into a DeepSeek Harness profile via git; there is still no npm publish fallback.

What users will see

• Git / path installs of @open-design/dsh-runtime produce a package whose declared entry points exist (dist/ built during install via prepare).
• DSH Desktop (and other consumers) can load open-design-runtime / open-design-startup without ERR_MODULE_NOT_FOUND from a missing build.

Surface area

• UI - new page / dialog / panel / menu item / setting / empty state in apps/web or apps/desktop (including Electron menu bar)
• Keyboard shortcut - new or changed
• CLI / env var - new od subcommand or flag, new tools-dev / tools-pack flag, or new OD_* env var
• API / contract - new /api/* endpoint, new SSE event, or changed shape in packages/contracts
• Extension point - new entry under skills/, design-systems/, design-templates/, or craft/, or change to the skills protocol
• i18n keys - added new translation keys (see TRANSLATIONS.md for the locale workflow)
• New top-level dependency - adding any new entry to the root package.json
• Default behavior change - changes what existing users experience without opting in
• None - internal packaging fix only (packages/dsh-runtime/package.json scripts)

Screenshots

N/A

Bug fix verification

• Test path that reproduces the bug: package had build but no lifecycle hook pnpm runs for git deps; after install, dist/index.js / dist/startup.js are absent while package.json main/exports still point at them (as reported in @open-design/dsh-runtime: git installs are broken — no prepare script, dist never built, sources stripped by files field #7471).
• Did the test go red on main and green on this branch? no — a red automated spec for install lifecycle was not cheap here (needs a full pnpm git-dep install of this package with its peer/dev toolchain).
• Verification instead: confirmed on main that packages/dsh-runtime/package.json scripts were only build / test / typecheck; this PR adds "prepare": "pnpm run build" so install-time lifecycle builds dist/ before the package is linked (same fix suggested in @open-design/dsh-runtime: git installs are broken — no prepare script, dist never built, sources stripped by files field #7471). Workspace / local pnpm run build behavior is unchanged.

Validation

• Inspected packages/dsh-runtime/package.json before/after; single-line scripts change only.
• Did not run full monorepo pnpm install / pnpm guard in this environment (sparse checkout of packages/dsh-runtime only; disk-constrained). Change is additive lifecycle wiring over the existing build script.

[lefarcen]

Thanks @VedantMadane — the #7471 packaging failure is easy to map from your write-up. I've routed this for review and linked it back to the issue so it doesn't get lost.

[PerishCode]

The git-install path still needs a packaging change that avoids preparing the whole monorepo. I reproduced the exact dependency install at this commit and the new hook expands into repository-wide installation work before this package can build.

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[PerishCode]

Avoid using prepare on this package while it is consumed from a subdirectory of the monorepo. With pnpm 10.33.2, an exact install of github:nexu-io/open-design#84c90101165ab75081d36b67dde4d8e00d865d8d&path:packages/dsh-runtime first fails unless the consumer allowlists this script; after allowlisting it, pnpm runs pnpm install against the fetched repository workspace. The reproduction selected all 29 workspace projects and installed 1,292 packages, then ran unrelated native postinstalls including onnxruntime-node; the runtime build had still not started after several minutes. This turns a small plugin dependency into a full repository/native-toolchain installation, so it can fail on the same constrained consumer environments this PR is intended to fix and executes much more third-party install code than the package requires. Please ship a self-contained prebuilt package instead (for example, publish/consume the existing packed artifact with dist, or provide an equivalent repository artifact that does not need a git prepare), and add an install-level regression that imports both the root and ./startup exports from that artifact.

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