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/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 {