fix(build-node): preserve Bun linker for node-pty install

[Hona]

Summary

• detect whether the current Bun install layout is hoisted or isolated before the follow-up bun install
• pass the same linker when installing @lydell/node-pty so the Windows GITHUB_ACTIONS path does not mix hoisted and isolated installs
• keep build-node.ts aligned with the existing install method instead of falling back to the monorepo default

Testing

• GITHUB_ACTIONS=true bun --cwd packages/opencode script/build-node.ts
• bun turbo typecheck (via pre-push hook)

[Copilot]

Pull request overview

This PR updates the packages/opencode Node build script to ensure that when it runs a follow-up bun install for @lydell/node-pty, it uses the same Bun linker mode (hoisted vs isolated) as the current workspace install layout—avoiding mixed install layouts (notably on Windows in GITHUB_ACTIONS).

Changes:

• Add linker-mode detection by inspecting workspace/root node_modules placement (with a fallback based on bun.lock + workspace config).
• Pass --linker=<detected> to the bun install ... @lydell/node-pty@... step in build-node.ts.

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