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

types.go

@@ -1,5 +1,7 @@
 package main
 
+import "time"
+
 // SessionState represents the current activity state of a Claude Code session.
 type SessionState int
 
@@ -32,11 +34,14 @@ type Process struct {
 
 // Session represents a Claude Code session enriched with JSONL metadata.
 type Session struct {
-	Process   Process
-	SessionID string
-	GitBranch string
-	State     SessionState
-	Preview   string // last lines of output
-	CwdPath   string // process cwd
-	Worktree  string // git worktree (may differ from cwd)
+	Process      Process
+	SessionID    string
+	GitBranch    string
+	State        SessionState
+	Preview      string     // last lines of output
+	CwdPath      string     // process cwd
+	Worktree     string     // git worktree (may differ from cwd)
+	Workspace    string     // i3 workspace number
+	Label        string     // user-assigned label
+	WaitingSince *time.Time // when session started waiting for input
 }