fix: build internal/config on Windows

internal/config/config.go

@@ -6,14 +6,16 @@ import (
 	"encoding/json"
 	"os"
 	"path/filepath"
-	"syscall"
 
 	"github.com/marcus/td/internal/models"
 )
 
 const configFile = ".todos/config.json"
 const lockFile = ".todos/config.json.lock"
 
+// withConfigLock serializes access to config.json using an OS file lock.
+// Implemented per-platform in lock_unix.go (flock) and lock_windows.go (LockFileEx).
+
 // Title validation defaults
 const (
 	DefaultTitleMinLength = 15
@@ -75,28 +77,6 @@ func Save(baseDir string, cfg *models.Config) error {
 	return os.Rename(tmpName, configPath)
 }
 
-// withConfigLock serializes access to config.json using flock
-func withConfigLock(baseDir string, fn func() error) error {
-	lockPath := filepath.Join(baseDir, lockFile)
-
-	if err := os.MkdirAll(filepath.Dir(lockPath), 0755); err != nil {
-		return err
-	}
-
-	f, err := os.OpenFile(lockPath, os.O_CREATE|os.O_RDWR, 0644)
-	if err != nil {
-		return err
-	}
-	defer f.Close()
-
-	if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX); err != nil {
-		return err
-	}
-	defer func() { _ = syscall.Flock(int(f.Fd()), syscall.LOCK_UN) }()
-
-	return fn()
-}
-
 // SetFocus sets the focused issue ID
 func SetFocus(baseDir string, issueID string) error {
 	return withConfigLock(baseDir, func() error {

internal/config/lock_unix.go

@@ -0,0 +1,31 @@
+//go:build unix
+
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+// withConfigLock serializes access to config.json using flock.
+func withConfigLock(baseDir string, fn func() error) error {
+	lockPath := filepath.Join(baseDir, lockFile)
+
+	if err := os.MkdirAll(filepath.Dir(lockPath), 0755); err != nil {
+		return err
+	}
+
+	f, err := os.OpenFile(lockPath, os.O_CREATE|os.O_RDWR, 0644)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX); err != nil {
+		return err
+	}
+	defer func() { _ = syscall.Flock(int(f.Fd()), syscall.LOCK_UN) }()
+
+	return fn()
+}

internal/config/lock_windows.go

@@ -0,0 +1,46 @@
+//go:build windows
+
+package config
+
+import (
+	"os"
+	"path/filepath"
+
+	"golang.org/x/sys/windows"
+)
+
+// withConfigLock serializes access to config.json using LockFileEx.
+func withConfigLock(baseDir string, fn func() error) error {
+	lockPath := filepath.Join(baseDir, lockFile)
+
+	if err := os.MkdirAll(filepath.Dir(lockPath), 0755); err != nil {
+		return err
+	}
+
+	f, err := os.OpenFile(lockPath, os.O_CREATE|os.O_RDWR, 0644)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	// LockFileEx with LOCKFILE_EXCLUSIVE_LOCK (blocking) locks the first byte
+	// of the file, which is sufficient for whole-file mutual exclusion since
+	// every contender uses the same offset/length.
+	ol := new(windows.Overlapped)
+	if err := windows.LockFileEx(
+		windows.Handle(f.Fd()),
+		windows.LOCKFILE_EXCLUSIVE_LOCK,
+		0, // reserved
+		1, // lock 1 byte
+		0, // high bits of length
+		ol,
+	); err != nil {
+		return err
+	}
+	defer func() {
+		ul := new(windows.Overlapped)
+		_ = windows.UnlockFileEx(windows.Handle(f.Fd()), 0, 1, 0, ul)
+	}()
+
+	return fn()
+}