fix: add Windows support for config file locking

[sctmike]

Summary

config.go uses syscall.Flock and syscall.LOCK_EX directly, which are POSIX-only and don't exist on Windows. This causes a build failure:

# github.com/marcus/td/internal/config
config.go:92:20: undefined: syscall.Flock
config.go:92:47: undefined: syscall.LOCK_EX
config.go:95:16: undefined: syscall.Flock
config.go:95:43: undefined: syscall.LOCK_UN

This was introduced in ac37b22 (PR #12), even though db/lock_windows.go and serve/portfile_windows.go already had the correct platform-specific pattern.

Fix

Split withConfigLock out of config.go into:

• config_lock_unix.go (//go:build unix) — original syscall.Flock implementation
• config_lock_windows.go (//go:build windows) — uses windows.LockFileEx / windows.UnlockFileEx from golang.org/x/sys/windows

This matches the existing pattern in db/ and serve/.

Test plan

• go build . succeeds on Windows (previously failed)
• td version runs correctly from native Windows binary
• Existing Unix builds unaffected (same code, just moved to build-tagged file)

[marcus]

Hey @sctmike! Starling here (AI assistant on the project). 👋

This is a solid fix — you've correctly identified that config.go was using syscall.Flock directly instead of the platform-specific pattern that db/ and serve/ already follow. The split into config_lock_unix.go / config_lock_windows.go with the right build tags is exactly the right approach.

Great that you verified go build and td version on native Windows — that's exactly the kind of confirmation we need for cross-platform fixes.

Flagging for Marcus to review and merge. Thanks for taking the time to track this down and submit the fix! ✦