Add local JSON daemon and route TUI/CLI through it.

A per-DataDir background process owns ticket JSON, history, and sync so
multiple TUI instances stay consistent; MCP token APIs are ready for phase 2.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/daemon/paths.go

@@ -0,0 +1,91 @@
+package daemon
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"syscall"
+)
+
+const (
+	sockName = "daemon.sock"
+	pidName  = "daemon.pid"
+	logName  = "daemon.log"
+)
+
+// Paths holds filesystem locations for one DataDir's daemon instance.
+type Paths struct {
+	RuntimeDir string
+	Socket     string
+	PIDFile    string
+	LogFile    string
+}
+
+// PathsForDataDir returns runtime paths unique to dataDir.
+func PathsForDataDir(dataDir string) (Paths, error) {
+	dataDir = filepath.Clean(dataDir)
+	sum := sha256.Sum256([]byte(dataDir))
+	id := hex.EncodeToString(sum[:8])
+
+	dir, err := mkdirRuntime(id, dataDir)
+	if err != nil {
+		return Paths{}, err
+	}
+	return Paths{
+		RuntimeDir: dir,
+		Socket:     filepath.Join(dir, sockName),
+		PIDFile:    filepath.Join(dir, pidName),
+		LogFile:    filepath.Join(dir, logName),
+	}, nil
+}
+
+func mkdirRuntime(id, dataDir string) (string, error) {
+	candidates := []string{}
+	if r := strings.TrimSpace(os.Getenv("XDG_RUNTIME_DIR")); r != "" {
+		candidates = append(candidates, filepath.Join(r, "task-master-"+id))
+	}
+	candidates = append(candidates,
+		filepath.Join(os.TempDir(), fmt.Sprintf("task-master-%d-%s", os.Getuid(), id)),
+		filepath.Join(dataDir, ".runtime"),
+	)
+	var last error
+	for _, dir := range candidates {
+		if err := os.MkdirAll(dir, 0o700); err != nil {
+			last = err
+			continue
+		}
+		return dir, nil
+	}
+	if last == nil {
+		last = fmt.Errorf("no runtime dir candidate")
+	}
+	return "", fmt.Errorf("create runtime dir: %w", last)
+}
+
+func writePID(path string, pid int) error {
+	return os.WriteFile(path, []byte(strconv.Itoa(pid)+"\n"), 0o600)
+}
+
+func readPID(path string) (int, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return 0, err
+	}
+	s := strings.TrimSpace(string(data))
+	if s == "" {
+		return 0, fmt.Errorf("empty pid file")
+	}
+	return strconv.Atoi(s)
+}
+
+func processAlive(pid int) bool {
+	if pid <= 0 {
+		return false
+	}
+	err := syscall.Kill(pid, 0)
+	return err == nil
+}