fix(ci): use dynamic bun cache path for cross-platform support

[Hona]

Summary

• Fixes bun install cache not being restored on Windows CI runners (Blacksmith and GitHub-hosted)
• Root cause: the cache path was hardcoded to ~/.bun/install/cache, but bun resolves the cache directory through a priority chain (BUN_INSTALL_CACHE_DIR > BUN_INSTALL > XDG_CACHE_HOME > USERPROFILE/HOME) — and ~ tilde expansion by actions/cache isn't guaranteed to match bun's actual resolution on all runner environments
• Fix: install bun first, then use bun pm cache to get the real cache directory and feed that into actions/cache

Details

From the bun source (src/install/PackageManager/PackageManagerDirectories.zig), fetchCacheDirectoryPath resolves the install cache through:

1. ``
2. bunfig.toml cache_directory
3. /install/cache/
4. /.bun/install/cache/
5. C:\Users\Lukem (POSIX) / %USERPROFILE% (Windows) /.bun/install/cache/
6. Fallback: node_modules/.bun-cache

bun pm cache returns the exact resolved path, so using it as the source of truth eliminates any mismatch.

[Copilot]

Pull request overview

Updates the repository’s Bun setup composite action to make Bun’s install-cache restoration reliable across CI environments (notably Windows runners) by deriving the cache directory from Bun itself rather than hardcoding a tilde-based path.

Changes:

• Moves Bun dependency caching to occur after Bun is installed.
• Adds a step that queries Bun’s resolved cache directory via bun pm cache and feeds it into actions/cache.

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