feat(tui-go): session list / resume view (E)

R opens a navigable overlay of recent sessions (GET /sessions: short id, status,
workflow/stage, relative age); enter resumes via POST /sessions/{id}/resume, which
replays onto the existing global /stream (no WS re-dial). Fetch/resume errors surface
in the overlay, never panic. stdlib-only (ws.Client.HTTPBase derives the base URL).

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

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

@@ -53,6 +53,7 @@ const (
 	OverlayConfig
 	OverlayStats
 	OverlayIdeas
+	OverlaySessions
 )
 
 // RouterEntry is one line in a session's conversation transcript.
@@ -259,6 +260,14 @@ type Model struct {
 	ideasIndex   int
 	ideasLoading bool
 
+	// session browser (OverlaySessions) — recent sessions fetched over HTTP from
+	// GET /sessions, so prior runs are reachable after a server restart + TUI reopen
+	// (the WS stream only carries live sessions, not the historical roster).
+	sessionList        []SessionSummary
+	sessionListIndex   int
+	sessionListLoading bool
+	sessionListErr     string
+
 	// command palette
 	paletteFilter string
 	paletteIndex  int