feat(tui): show session workspace (cwd) in the status bar

SessionWorkspaceBoundEvent already recorded the bound workspace root but
it was never surfaced. Map it to a new session.workspace_bound frame
(sealed ServerMessage variant, auto-registered; re-emitted on reconnect
via the snapshot replay), carry it on Session.WorkspaceRoot, and show it
home-abbreviated in the status bar (⌂ ~/Programs/correx). Golden test
pins the decode.

apps/tui-go/internal/app/model.go

@@ -98,19 +98,20 @@ type Approval struct {
 
 // Session is the UI's view of one server session.
 type Session struct {
-	ID           string
-	Status       string
-	WorkflowID   string
-	Name         string
-	LastEventAt  int64
-	CurrentStage string
-	LastOutput   string
-	LastResponse string
-	Tools        []ToolRecord
-	ToolsByStage map[string][]ManifestTool
-	Events       []EventEntry
-	Pending      *Approval
-	Active       bool // an inference/tool is in flight (drives the spinner)
+	ID            string
+	Status        string
+	WorkflowID    string
+	Name          string
+	LastEventAt   int64
+	CurrentStage  string
+	WorkspaceRoot string // bound cwd, from session.workspace_bound
+	LastOutput    string
+	LastResponse  string
+	Tools         []ToolRecord
+	ToolsByStage  map[string][]ManifestTool
+	Events        []EventEntry
+	Pending       *Approval
+	Active        bool // an inference/tool is in flight (drives the spinner)
 }
 
 // Workflow is a launchable workflow advertised by the server.