feat(tui): surface plane-2 rationale in the approval band

The plane-2 tool-call assessor records verified preconditions
("[PATH_OUTSIDE_WORKSPACE] …") and the server already ships them in
RiskSummaryDto.rationale, but the Go TUI's RiskSummaryDto had no
Rationale field — so the justification was silently dropped at decode
and the gate showed only an opaque tier.

Decode the rationale, carry it on Approval, and render it under the
header in the approval band (warn-marked, above the diff). This closes
the last deferred item of the plane-2 slice-1 plan: the assessment
surfaced to the approval UX. Golden test pins the rationale decode.

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

@@ -75,9 +75,12 @@ func PreviewFrame(kind string, w, h int) string {
 			s.CurrentStage = "write_script"
 			s.Events = sampleEvents()
 			s.Pending = &Approval{
-				RequestID: "req-1", SessionID: "04a546aa", Tier: "T3", Risk: "HIGH",
+				RequestID: "req-1", SessionID: "04a546aa", Tier: "T3", Risk: "MEDIUM",
 				ToolName: "file_write",
 				Preview:  "--- a/healthcheck.sh\n+++ b/healthcheck.sh\n@@ -0,0 +1,4 @@\n+#!/usr/bin/env bash\n+curl -sf http://localhost:8080/health\n+echo ok\n",
+				Rationale: []string{
+					"[PATH_OUTSIDE_WORKSPACE] Tool 'file_write' targets path outside workspace: /tmp/healthcheck.sh",
+				},
 			}
 		}
 

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

@@ -91,6 +91,9 @@ type Approval struct {
 	Risk      string
 	ToolName  string
 	Preview   string
+	// Rationale holds the plane-2 verified preconditions ("[PATH_OUTSIDE_WORKSPACE] …")
+	// that justify the gate — shown in the approval band instead of an opaque tier.
+	Rationale []string
 }
 
 // Session is the UI's view of one server session.

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

@@ -56,11 +56,14 @@ func mbg(t Theme, s string, fg lipgloss.Color) string {
 // never crowds out the session above (longer diffs spill to the ^x fullscreen
 // view). Layout: 2 borders + header + blank + diff rows + blank + action row.
 func (m Model) approvalBandHeight() int {
-	rows := 0
+	rows, rat := 0, 0
 	if s := m.session(m.selectedID); s != nil && s.Pending != nil {
 		rows = len(parseUnifiedDiff(s.Pending.Preview))
+		if n := len(s.Pending.Rationale); n > 0 {
+			rat = n + 1 // rationale lines + trailing blank
 		}
-	h := 2 + 1 + 1 + rows + 1 + 1
+	}
+	h := 2 + 1 + 1 + rat + rows + 1 + 1
 	if maxH := m.height * 55 / 100; h > maxH {
 		h = maxH
 	}
@@ -103,7 +106,11 @@ func (m Model) renderApprovalBand(h int) string {
 		lipgloss.NewStyle().Foreground(riskColor).Background(t.P.Bg).Bold(true).Render(strings.ToUpper(a.Risk))
 
 	innerH := h - 2
-	diffH := innerH - 4 // header, blank, blank, action row
+	ratBlock := 0
+	if len(a.Rationale) > 0 {
+		ratBlock = len(a.Rationale) + 1 // rationale lines + trailing blank
+	}
+	diffH := innerH - 4 - ratBlock // header, blank, blank, action row, + rationale block
 	if diffH < 1 {
 		diffH = 1
 	}
@@ -111,8 +118,16 @@ func (m Model) renderApprovalBand(h int) string {
 
 	body := make([]string, 0, innerH)
 	body = append(body, m.justify(leftHdr, rightHdr, textW, t.P.Bg), "")
+	for _, r := range a.Rationale {
+		marker := lipgloss.NewStyle().Foreground(t.P.Warn).Background(t.P.Bg).Render("▲ ")
+		body = append(body, marker+t.span(truncate(r, textW-2), t.P.Dim))
+	}
+	if len(a.Rationale) > 0 {
+		body = append(body, "")
+	}
+	diffStart := len(body)
 	body = append(body, m.renderSplitDiff(rows, textW, diffH, 0)...)
-	for len(body) < 2+diffH {
+	for len(body) < diffStart+diffH {
 		body = append(body, "")
 	}
 	body = append(body, "", m.approvalActions())

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

@@ -261,8 +261,10 @@ func (m *Model) onSessionAnnounced(msg protocol.ServerMessage) {
 
 func (m *Model) onApprovalRequired(msg protocol.ServerMessage) {
 	risk := "unknown"
+	var rationale []string
 	if msg.RiskSummary != nil {
 		risk = msg.RiskSummary.Level
+		rationale = msg.RiskSummary.Rationale
 	}
 	info := &Approval{
 		RequestID: msg.RequestID,
@@ -271,6 +273,7 @@ func (m *Model) onApprovalRequired(msg protocol.ServerMessage) {
 		Risk:      risk,
 		ToolName:  deref(&msg.ToolName),
 		Preview:   derefp(msg.Preview),
+		Rationale: rationale,
 	}
 	if s := m.session(msg.SessionID); s != nil {
 		s.Pending = info

apps/tui-go/internal/protocol/golden_test.go

@@ -58,6 +58,22 @@ func TestDecodeGoldenServerFrames(t *testing.T) {
 				}
 			},
 		},
+		{
+			// Plane-2 rationale (verified preconditions) must survive decode — it is the
+			// justification shown in the approval band, not the opaque tier.
+			name:      "approval.required carries plane-2 rationale",
+			json:      `{"type":"approval.required","sessionId":"s1","requestId":"req-1","tier":"T3","riskSummary":{"level":"MEDIUM","factors":[],"recommendedAction":"PROMPT_USER","rationale":["[PATH_OUTSIDE_WORKSPACE] outside: /tmp/x"]},"toolName":"file_write","preview":"--- a/x\n+++ b/x","sequence":12,"sessionSequence":3}`,
+			wantType:  TypeApprovalRequired,
+			eventBear: true,
+			check: func(t *testing.T, m ServerMessage) {
+				if m.RiskSummary == nil {
+					t.Fatalf("approval.required: riskSummary decoded nil")
+				}
+				if len(m.RiskSummary.Rationale) != 1 || m.RiskSummary.Rationale[0] != "[PATH_OUTSIDE_WORKSPACE] outside: /tmp/x" {
+					t.Fatalf("rationale not decoded: %+v", m.RiskSummary)
+				}
+			},
+		},
 		{
 			name:      "snapshot_complete (non-event control frame)",
 			json:      `{"type":"snapshot_complete"}`,

apps/tui-go/internal/protocol/protocol.go

@@ -110,6 +110,7 @@ type RiskSummaryDto struct {
 	Level             string   `json:"level"`
 	Factors           []string `json:"factors"`
 	RecommendedAction string   `json:"recommendedAction"`
+	Rationale         []string `json:"rationale"`
 }
 
 type AssessedIssueDto struct {