feat(tui): render structured artifacts as a summary, not raw JSON

Approval-card and preview JSON artifacts (the freestyle execution_plan, the analyst's analysis) rendered as a raw JSON dump. Add a JSON-aware preview: a scannable goal + numbered stage list for execution_plan, indented key/value lines for other objects, with light highlighting. Non-JSON previews fall back to the raw line view unchanged.

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

@@ -94,19 +94,29 @@ func previewPlainLines(s string) []string {
 	return strings.Split(strings.TrimRight(s, "\n"), "\n")
 }
 
+// previewDisplayLines is the single source of truth for how a non-diff preview is laid out:
+// a scannable summary for structured (JSON) artifacts, raw lines otherwise. Both the renderer
+// and previewRowCount go through it so band sizing and scroll bounds match what's drawn.
+func previewDisplayLines(s string) []string {
+	if lines, ok := artifactDisplayLines(s); ok {
+		return lines
+	}
+	return previewPlainLines(s)
+}
+
 // previewRowCount is the number of content rows a preview occupies — diff rows
 // for a real diff, plain lines otherwise. Drives band sizing and scroll bounds.
 func previewRowCount(s string) int {
 	if isUnifiedDiff(s) {
 		return len(parseUnifiedDiff(s))
 	}
-	return len(previewPlainLines(s))
+	return len(previewDisplayLines(s))
 }
 
 // renderPreviewLines renders non-diff preview text as dim plain lines (single
 // column), up to maxRows starting at off, each truncated to width.
 func (m Model) renderPreviewLines(content string, width, maxRows, off int) []string {
-	lines := previewPlainLines(content)
+	lines := previewDisplayLines(content)
 	if off < 0 {
 		off = 0
 	}
@@ -119,11 +129,34 @@ func (m Model) renderPreviewLines(content string, width, maxRows, off int) []str
 	}
 	out := make([]string, 0, end-off)
 	for _, ln := range lines[off:end] {
-		out = append(out, m.theme.span(truncate(ln, width), m.theme.P.Dim))
+		out = append(out, m.colorizePreviewLine(ln, width))
 	}
 	return out
 }
 
+// colorizePreviewLine highlights a summarized-artifact line: flush-left section headers (Goal,
+// Plan · N) in accent, the "→ produces" target picked out, everything else dim — so the structure
+// reads at a glance without a full JSON syntax highlighter. Plain (raw) previews stay uniformly dim.
+func (m Model) colorizePreviewLine(ln string, w int) string {
+	t := m.theme
+	ln = truncate(ln, w)
+	if ln == "" {
+		return ""
+	}
+	// Section header: no leading indent (e.g. "Goal", "Plan · 4 stages").
+	if ln[0] != ' ' {
+		return lipgloss.NewStyle().Foreground(t.P.Accent2).Background(t.P.Bg).Bold(true).Render(ln)
+	}
+	// Stage head: pick out the "→ produces" target after truncation so widths stay bounded.
+	if i := strings.Index(ln, " → "); i >= 0 {
+		head := t.span(ln[:i], t.P.FgStrong)
+		arrow := t.span(" → ", t.P.Faint)
+		prod := lipgloss.NewStyle().Foreground(t.P.OK).Background(t.P.Bg).Render(ln[i+len(" → "):])
+		return head + arrow + prod
+	}
+	return t.span(ln, t.P.Dim)
+}
+
 // hunkStarts reads the starting old/new line numbers from an @@ -a,b +c,d @@ marker.
 func hunkStarts(h string) (int, int) {
 	old, newer := 1, 1