feat(02-01): implement protocol types, SessionRegistry, and LabelStore

- Enrich Session with Workspace, Label, WaitingSince fields
- Protocol types: Request, Response, SessionInfo, SwitchArgs, LabelArgs
- SessionRegistry with WaitingSince transition tracking
- LabelStore with JSON file persistence

protocol.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"encoding/json"
+	"time"
+)
+
+// Request is the JSON message sent by a client to the daemon over the Unix socket.
+type Request struct {
+	Action string          `json:"action"` // "list", "switch", "label", "stop"
+	Args   json.RawMessage `json:"args,omitempty"`
+}
+
+// Response is the JSON message sent back by the daemon.
+type Response struct {
+	OK       bool          `json:"ok"`
+	Error    string        `json:"error,omitempty"`
+	Sessions []SessionInfo `json:"sessions,omitempty"`
+}
+
+// SessionInfo is the wire format for a session in IPC responses.
+type SessionInfo struct {
+	PID          int        `json:"pid"`
+	SessionID    string     `json:"session_id"`
+	Cwd          string     `json:"cwd"`
+	GitBranch    string     `json:"git_branch"`
+	State        string     `json:"state"`
+	Preview      string     `json:"preview"`
+	Workspace    string     `json:"workspace"`
+	Label        string     `json:"label,omitempty"`
+	WaitingSince *time.Time `json:"waiting_since,omitempty"`
+}
+
+// SwitchArgs carries the query for workspace switching.
+type SwitchArgs struct {
+	Query string `json:"query"`
+}
+
+// LabelArgs carries the session ID and label for the label action.
+type LabelArgs struct {
+	SessionID string `json:"session_id"`
+	Label     string `json:"label"`
+}