feat: Add atomic write pattern for state files (gt-wled7)

Prevents data loss from concurrent/interrupted state file writes by using
atomic write pattern (write to .tmp, then rename).

Changes:
- Add internal/util package with AtomicWriteJSON/AtomicWriteFile helpers
- Update witness/manager.go saveState to use atomic writes
- Update refinery/manager.go saveState to use atomic writes
- Update crew/manager.go saveState to use atomic writes
- Update daemon/types.go SaveState to use atomic writes
- Update polecat/namepool.go Save to use atomic writes
- Add comprehensive tests for atomic write utilities

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/util/atomic.go

@@ -0,0 +1,41 @@
+// Package util provides common utilities for Gas Town.
+package util
+
+import (
+	"encoding/json"
+	"os"
+)
+
+// AtomicWriteJSON writes JSON data to a file atomically.
+// It first writes to a temporary file, then renames it to the target path.
+// This prevents data corruption if the process crashes during write.
+// The rename operation is atomic on POSIX systems.
+func AtomicWriteJSON(path string, v interface{}) error {
+	data, err := json.MarshalIndent(v, "", "  ")
+	if err != nil {
+		return err
+	}
+	return AtomicWriteFile(path, data, 0644)
+}
+
+// AtomicWriteFile writes data to a file atomically.
+// It first writes to a temporary file, then renames it to the target path.
+// This prevents data corruption if the process crashes during write.
+// The rename operation is atomic on POSIX systems.
+func AtomicWriteFile(path string, data []byte, perm os.FileMode) error {
+	tmpFile := path + ".tmp"
+
+	// Write to temp file
+	if err := os.WriteFile(tmpFile, data, perm); err != nil {
+		return err
+	}
+
+	// Atomic rename (on POSIX systems)
+	if err := os.Rename(tmpFile, path); err != nil {
+		// Clean up temp file on failure
+		_ = os.Remove(tmpFile)
+		return err
+	}
+
+	return nil
+}

internal/util/atomic_test.go

@@ -0,0 +1,88 @@
+package util
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestAtomicWriteJSON(t *testing.T) {
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "test.json")
+
+	// Test basic write
+	data := map[string]string{"key": "value"}
+	if err := AtomicWriteJSON(testFile, data); err != nil {
+		t.Fatalf("AtomicWriteJSON error: %v", err)
+	}
+
+	// Verify file exists
+	if _, err := os.Stat(testFile); os.IsNotExist(err) {
+		t.Fatal("File was not created")
+	}
+
+	// Verify temp file was cleaned up
+	tmpFile := testFile + ".tmp"
+	if _, err := os.Stat(tmpFile); !os.IsNotExist(err) {
+		t.Fatal("Temp file was not cleaned up")
+	}
+
+	// Read and verify content
+	content, err := os.ReadFile(testFile)
+	if err != nil {
+		t.Fatalf("ReadFile error: %v", err)
+	}
+	if string(content) != "{\n  \"key\": \"value\"\n}" {
+		t.Fatalf("Unexpected content: %s", content)
+	}
+}
+
+func TestAtomicWriteFile(t *testing.T) {
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "test.txt")
+
+	// Test basic write
+	data := []byte("hello world")
+	if err := AtomicWriteFile(testFile, data, 0644); err != nil {
+		t.Fatalf("AtomicWriteFile error: %v", err)
+	}
+
+	// Verify content
+	content, err := os.ReadFile(testFile)
+	if err != nil {
+		t.Fatalf("ReadFile error: %v", err)
+	}
+	if string(content) != "hello world" {
+		t.Fatalf("Unexpected content: %s", content)
+	}
+
+	// Verify temp file was cleaned up
+	tmpFile := testFile + ".tmp"
+	if _, err := os.Stat(tmpFile); !os.IsNotExist(err) {
+		t.Fatal("Temp file was not cleaned up")
+	}
+}
+
+func TestAtomicWriteOverwrite(t *testing.T) {
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "test.json")
+
+	// Write initial content
+	if err := AtomicWriteJSON(testFile, "first"); err != nil {
+		t.Fatalf("First write error: %v", err)
+	}
+
+	// Overwrite with new content
+	if err := AtomicWriteJSON(testFile, "second"); err != nil {
+		t.Fatalf("Second write error: %v", err)
+	}
+
+	// Verify new content
+	content, err := os.ReadFile(testFile)
+	if err != nil {
+		t.Fatalf("ReadFile error: %v", err)
+	}
+	if string(content) != "\"second\"" {
+		t.Fatalf("Unexpected content: %s", content)
+	}
+}