feat: always post fresh review, supersede old with collapsed body

Closes #34

- Remove reviewUnchanged() skip logic — every push gets a fresh review
- Remove edit-in-place (PATCH same body) — always POST new
- Supersede old review: PATCH with struck-through banner + collapsed
  original body in <details> for historical reference
- Add commit footer to every review: 'Evaluated against <sha>'
- Remove --update-existing flag (no longer needed)
- Add CommitID field to Review struct
- Add TestBuildSupersededBody tests

cmd/review-bot/main_test.go

@@ -56,82 +56,48 @@ func makeReview(id int64, login, state string, stale bool, body string) gitea.Re
 	return r
 }
 
-func TestReviewUnchanged(t *testing.T) {
-	tests := []struct {
-		name     string
-		existing []gitea.Review
-		newBody  string
-		newEvent string
-		sentinel string
-		want     bool
-	}{
-		{
-			name:     "no existing review",
-			existing: nil,
-			newBody:  "new review",
-			newEvent: "APPROVED",
-			sentinel: "<!-- review-bot:sonnet -->",
-			want:     false,
-		},
-		{
-			name: "identical body and state",
-			existing: []gitea.Review{
-				makeReview(100, "bot", "APPROVED", false, "same body\n<!-- review-bot:sonnet -->"),
-			},
-			newBody:  "same body\n<!-- review-bot:sonnet -->",
-			newEvent: "APPROVED",
-			sentinel: "<!-- review-bot:sonnet -->",
-			want:     true,
-		},
-		{
-			name: "same body but different state",
-			existing: []gitea.Review{
-				makeReview(100, "bot", "APPROVED", false, "body\n<!-- review-bot:sonnet -->"),
-			},
-			newBody:  "body\n<!-- review-bot:sonnet -->",
-			newEvent: "REQUEST_CHANGES",
-			sentinel: "<!-- review-bot:sonnet -->",
-			want:     false,
-		},
-		{
-			name: "different body same state",
-			existing: []gitea.Review{
-				makeReview(100, "bot", "APPROVED", false, "old body\n<!-- review-bot:sonnet -->"),
-			},
-			newBody:  "new body\n<!-- review-bot:sonnet -->",
-			newEvent: "APPROVED",
-			sentinel: "<!-- review-bot:sonnet -->",
-			want:     false,
-		},
-		{
-			name: "stale review with same body (should still post)",
-			existing: []gitea.Review{
-				makeReview(100, "bot", "APPROVED", true, "same\n<!-- review-bot:sonnet -->"),
-			},
-			newBody:  "same\n<!-- review-bot:sonnet -->",
-			newEvent: "APPROVED",
-			sentinel: "<!-- review-bot:sonnet -->",
-			want:     false,
-		},
-		{
-			name: "different sentinel (not our review)",
-			existing: []gitea.Review{
-				makeReview(100, "bot", "APPROVED", false, "body\n<!-- review-bot:gpt -->"),
-			},
-			newBody:  "body\n<!-- review-bot:sonnet -->",
-			newEvent: "APPROVED",
-			sentinel: "<!-- review-bot:sonnet -->",
-			want:     false,
-		},
-	}
 
-	for _, tc := range tests {
-		t.Run(tc.name, func(t *testing.T) {
-			got := reviewUnchanged(tc.existing, tc.newBody, tc.newEvent, tc.sentinel)
-			if got != tc.want {
-				t.Errorf("reviewUnchanged() = %v, want %v", got, tc.want)
-			}
-		})
+func TestBuildSupersededBody(t *testing.T) {
+	original := "# Review\n\nLooks good.\n\n<!-- review-bot:sonnet -->"
+	sentinel := "<!-- review-bot:sonnet -->"
+
+	result := buildSupersededBody(original, "abcdef1234567890", sentinel)
+
+	// Should contain the struck-through banner
+	if !strings.Contains(result, "~~Original review~~") {
+		t.Error("missing struck-through banner")
+	}
+	// Should contain superseded notice
+	if !strings.Contains(result, "**Superseded**") {
+		t.Error("missing superseded notice")
+	}
+	// Should contain collapsed original
+	if !strings.Contains(result, "<details>") {
+		t.Error("missing details/collapse")
+	}
+	// Should contain short commit SHA
+	if !strings.Contains(result, "abcdef12") {
+		t.Error("missing short SHA")
+	}
+	// Should NOT contain full SHA
+	if strings.Contains(result, "abcdef1234567890") {
+		t.Error("should truncate SHA to 8 chars")
+	}
+	// Should contain the original body inside details
+	if !strings.Contains(result, original) {
+		t.Error("original body not preserved in collapsed section")
+	}
+	// Should end with sentinel
+	if !strings.Contains(result, sentinel) {
+		t.Error("missing sentinel")
+	}
+}
+
+func TestBuildSupersededBodyShortSHA(t *testing.T) {
+	// Short SHA should pass through without panic
+	result := buildSupersededBody("body", "abc", "<!-- review-bot:x -->")
+	if !strings.Contains(result, "abc") {
+		t.Error("short SHA not preserved")
 	}
 }