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/refinery/manager.go

@@ -18,6 +18,7 @@ import (
 	"github.com/steveyegge/gastown/internal/mail"
 	"github.com/steveyegge/gastown/internal/rig"
 	"github.com/steveyegge/gastown/internal/tmux"
+	"github.com/steveyegge/gastown/internal/util"
 )
 
 // Common errors
@@ -80,19 +81,14 @@ func (m *Manager) loadState() (*Refinery, error) {
 	return &ref, nil
 }
 
-// saveState persists refinery state to disk.
+// saveState persists refinery state to disk using atomic write.
 func (m *Manager) saveState(ref *Refinery) error {
 	dir := filepath.Dir(m.stateFile())
 	if err := os.MkdirAll(dir, 0755); err != nil {
 		return err
 	}
 
-	data, err := json.MarshalIndent(ref, "", "  ")
-	if err != nil {
-		return err
-	}
-
-	return os.WriteFile(m.stateFile(), data, 0644)
+	return util.AtomicWriteJSON(m.stateFile(), ref)
 }
 
 // Status returns the current refinery status.