fix: Enable Windows builds and fix bun+pnpm install on Windows

[Hona]

Fixes #3363 - bun install -g opencode-ai now works on Windows
Fixes #4195 - pnpm install -g opencode-ai now works on Windows

Problem

OpenCode could not be built or installed on Windows using bun due to three critical issues:

1. Build script platform mismatch: The build script used "windows" as the platform identifier, but process.platform returns "win32" on Windows. This caused:

   • The --single flag to fail (built wrong platform or nothing at all)
   • Package names like opencode-windows-x64 that didn't match the expected opencode-win32-x64

2. Windows path separator bug: The build script used path.relative() which returns backslashes on Windows (\), but the bundled code expected forward slashes (/). This
   caused syntax errors and made all Windows builds fail.

3. Bun install failure: The postinstall script skipped all processing when detecting bun on Windows, causing bun to generate a default #!/bin/sh wrapper script that failed
   with:
   error: interpreter executable "/bin/sh" not found in %PATH%

Solution

1. Platform naming consistency

• Changed platform identifier from "windows" to "win32" in:
  • build.ts: Target definitions and package.json generation
  • postinstall.mjs: Platform detection logic
• This ensures consistency with Node.js's process.platform value

Warning

I'm not sure how this would interact with old installs -> upgrade paths

2. Path separator normalization

• Added .replace(/\\/g, "/") to normalize Windows backslashes to forward slashes in bundled paths
• Fixes the OTUI_TREE_SITTER_WORKER_PATH define that was causing build failures

3. Bun support on Windows

• Added explicit bun detection and handling in postinstall.mjs
• For bun on Windows: copies the .exe directly to the bin directory (bun doesn't need wrapper scripts)
• For npm on Windows: continues to use npm rebuild to regenerate CMD/PS1 wrappers
• For other package managers on Windows: gracefully skips with a clear message

Testing

Verified the following scenarios work correctly:

• ✅ bun run script/build.ts --single on Windows (now builds opencode-win32-x64)
• ✅ Full build on Windows without errors
• ✅ bun install -g opencode-ai on Windows (no more /bin/sh error)
• ✅ npm install -g opencode-ai on Windows (CMD wrappers still generated correctly)
• ✅ Postinstall script correctly detects and handles npm vs bun on Windows

[Copilot]

Pull Request Overview

This PR fixes critical Windows build and installation issues for OpenCode by ensuring platform identifier consistency and proper path handling across the build and postinstall scripts.

• Unified platform identifier to use "win32" instead of "windows" throughout the codebase to match Node.js's process.platform
• Fixed Windows path separator issues by normalizing backslashes to forward slashes in bundled paths
• Added explicit bun support on Windows by copying the executable directly instead of relying on wrapper scripts

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/opencode/script/postinstall.mjs	Updated platform detection to use "win32" consistently, added bun-specific binary installation logic for Windows that copies the .exe directly, and improved user agent detection with safe fallback
packages/opencode/script/build.ts	Changed platform identifier from "windows" to "win32" in target definitions, normalized Windows path separators in the tree-sitter worker path, and simplified package.json OS field generation

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[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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