feat(tui-go): transcript message-jump + OSC52 copy

ctrl+↑/↓ jumps between your sent (user) messages in the transcript: ctrl+↑ from the
bottom selects the most recent, steps to older ones (clamping); ctrl+↓ steps back and
drops to tail-follow past the last. The selected message is highlighted and scrolled
into view (routerRows tracks per-message row offsets and windows to the selection).

`y` yanks the selected message — or the latest turn when none is selected — to the
system clipboard over OSC52 (go-osc52, tmux-wrapped under $TMUX), which is SSH/Termius-
safe unlike a mouse drag. A brief "✓ copied" footer note animates out (and self-stops
the tick). esc drops the selection; switching sessions clears it. Tests cover the
user-message jump/clamp/tail and the copy-text selection.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -241,6 +241,11 @@ type Model struct {
 	routerMessages  map[string][]RouterEntry
 	routerConnected bool
 	chatMode        string
+	// transcriptSel is the index (into the selected session's transcript) of the message
+	// the operator has navigated to with ctrl+↑/↓; -1 = no selection (tail-follow). A
+	// selected message is highlighted, scrolled into view, and is what `y` copies.
+	transcriptSel int
+	copiedFlash   int // frame at which a copy happened, for a brief "copied" footer note
 
 	// provider
 	currentModel string
@@ -353,6 +358,7 @@ func NewModel(client *ws.Client) Model {
 		providerType:    "LOCAL",
 		snapshotPhase:   true,
 		eventStripShown: true,
+		transcriptSel:   -1,
 	}
 }