fix: replace all non-ASCII characters with safe ASCII equivalents

Replace box-drawing characters (U+2500) in session log with ASCII dashes.
Replace em dashes (U+2014) in CLAUDE.md template with double hyphens.
Remove em dash from comment in run.go.
Add ASCII-guard tests for session log output and seed templates.
Prevents mojibake on Windows terminals that misinterpret UTF-8 as CP1252.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/session/log.go

@@ -40,7 +40,7 @@ func FormatSessionEntry(info *SessionInfo) string {
 	duration := info.EndTime.Sub(info.StartTime)
 	timeFmt := "2006-01-02 15:04:05"
 
-	fmt.Fprintf(&b, "── Session %s ──\n", info.StartTime.Format(timeFmt))
+	fmt.Fprintf(&b, "-- Session %s --\n", info.StartTime.Format(timeFmt))
 	fmt.Fprintf(&b, "Agent:    %s\n", info.Agent)
 	fmt.Fprintf(&b, "Mode:     %s\n", info.Mode)
 	fmt.Fprintf(&b, "Start:    %s\n", info.StartTime.Format(timeFmt))
@@ -96,7 +96,7 @@ func FormatSessionEntry(info *SessionInfo) string {
 		b.WriteString("(No changes detected)\n")
 	}
 
-	b.WriteString("────────────────────────────────\n")
+	b.WriteString("--------------------------------\n")
 
 	return b.String()
 }

internal/session/log_test.go

@@ -127,8 +127,8 @@ func TestAppendSessionLog(t *testing.T) {
 	content := string(data)
 
 	// Should contain both sessions
-	if strings.Count(content, "── Session") != 2 {
-		t.Errorf("expected 2 session entries, got %d", strings.Count(content, "── Session"))
+	if strings.Count(content, "-- Session") != 2 {
+		t.Errorf("expected 2 session entries, got %d", strings.Count(content, "-- Session"))
 	}
 }
 
@@ -167,3 +167,27 @@ func TestManifestCleanupOnSuccess(t *testing.T) {
 		t.Error("manifest should be cleaned up after successful logging")
 	}
 }
+
+func TestSessionLogIsASCII(t *testing.T) {
+	start := time.Date(2026, 4, 5, 14, 30, 22, 0, time.UTC)
+	end := time.Date(2026, 4, 5, 15, 45, 10, 0, time.UTC)
+
+	info := &SessionInfo{
+		Agent:     "claude",
+		Mode:      "local",
+		StartTime: start,
+		EndTime:   end,
+		Diff: &ManifestDiff{
+			Added:    []string{"output/result.md"},
+			Modified: []string{"notes.md"},
+		},
+	}
+
+	entry := FormatSessionEntry(info)
+	for i, b := range []byte(entry) {
+		if b > 127 {
+			t.Errorf("non-ASCII byte 0x%02x at position %d in session log output", b, i)
+			break
+		}
+	}
+}

internal/session/run.go

@@ -33,7 +33,7 @@ func Run(opts LaunchOpts) error {
 	startManifest, err := CaptureManifest(opts.WsDir)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "[ctask] warning: failed to capture start manifest: %v\n", err)
-		// Continue anyway — never block the user
+		// Continue anyway -- never block the user
 	}
 
 	if startManifest != nil {