Windows path support for Supabase auth store

[jumski]

Summary

The Supabase plugin's auth store (src/server/store.ts) has not been tested or validated on Windows. OpenCode CLI supports Windows, so the plugin should work there too.

Known discrepancies

• resolveStoreRoot() L27 uses worktree === dirname(worktree) to reject filesystem root paths. On Windows, path.dirname("C:\\") returns "C:\\" (same behavior as POSIX /), but UNC roots like \\server\share are untested. Node's path functions are platform-specific — running POSIX path.dirname on a Windows path produces wrong results and vice versa.
• The ..${sep} guard on L34 assumes POSIX sep === "/". On Windows sep === "\\".
• No Windows CI or cross-platform path tests exist yet.

Acceptance criteria

• Add Windows-style path scenarios to test suite (using path.win32 or CI on Windows)
• Evaluate parse(worktree).root === worktree as a cross-platform root guard
• Verify UNC path, drive-letter root, long-path (\\?\) handling
• Add Windows CI or at minimum path.win32 unit tests

References

• Original bug: worktree="/" caused EROFS on auth callback (9fd8d5a)
• Hardening: fa92b13, f4c1c9c
• Code: src/server/store.ts:20-40