fix(core): bypass Windows Git lookup

[opencode-agent]

Summary

• resolve the Git executable once on Windows through a shared Core helper
• use the resolved executable for repository, worktree, snapshot, and VCS operations
• bypass cross-spawn lookup for absolute native Windows .exe and .com commands
• preserve existing lookup behavior for non-Windows and shell-backed commands

Verification

• bun typecheck in packages/util
• bun typecheck in packages/core
• bun test test/git.test.ts test/vcs.test.ts in packages/core (12 passed)
• git diff --check

Requested by: @neriousy (Filip via Slack)