fix(harness): allow OpenCode and Grok Build install scripts (#19408)

## Background

pnpm 11 rejects dependency build scripts unless they are explicitly
allowed. The pinned `opencode-ai@1.18.3` and
`@xai-official/grok@0.2.111` CLIs require postinstall scripts, so their
sandbox bootstrap fails with `ERR_PNPM_IGNORED_BUILDS` before either
harness can start.

## Summary

- add exact-version `allowBuilds` policies to the OpenCode and Grok
Build bootstrap artifacts
- let ACP `npm-locked` sources carry an optional `pnpmWorkspaceYaml`
artifact
- include the ACP workspace artifact in implementation identity so
policy changes invalidate incompatible lifecycle state
- document locked-source workspace configuration and persisted-artifact
behavior
- publish patch changesets for `@ai-sdk/harness-acp`,
`@ai-sdk/harness-grok-build`, and `@ai-sdk/harness-opencode`

## End-to-End Verification

- installed the pinned OpenCode bridge dependencies from a clean
temporary directory with pnpm 11.0.0; `opencode-ai` postinstall
completed
- installed the pinned Grok Build implementation dependencies from a
clean temporary directory with pnpm 11.0.0; `@xai-official/grok`
postinstall completed

## Checklist

- [x] All commits are signed (PRs with unsigned commits cannot be
merged)
- [x] Tests have been added / updated (for bug fixes / features)
- [x] Documentation has been added / updated (for bug fixes / features)
- [x] A _patch_ changeset for relevant packages has been added (for bug
fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)