chore: codebase cleanup — remove dead code, consolidate imports, extract constants

- Remove 3 unused exported constants (TRIM_MESSAGES_TO, MAX_TERMINAL_COLS, MAX_TERMINAL_ROWS)
- Consolidate 8 direct util imports into barrel imports (./utils/index.js)
- Extract magic number 8191 to FILE_PEEK_BYTES constant in buffer-limits.ts
- Add explanatory comments to 9 undocumented .catch(() => {}) handlers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/plan-orchestrator.ts

@@ -521,7 +521,7 @@ export class PlanOrchestrator {
     } finally {
       // Always clean up session and progress interval — centralizing here
       // prevents the race where cancel() and catch both try to manage the set
-      await session.stop().catch(() => {});
+      await session.stop().catch(() => {}); // Ignore - session cleanup is best-effort in finally block
       this.runningSessions.delete(session);
       clearInterval(progressInterval);
     }
@@ -651,7 +651,7 @@ export class PlanOrchestrator {
     } finally {
       // Always clean up session and progress interval — centralizing here
       // prevents the race where cancel() and catch both try to manage the set
-      await session.stop().catch(() => {});
+      await session.stop().catch(() => {}); // Ignore - session cleanup is best-effort in finally block
       this.runningSessions.delete(session);
       clearInterval(progressInterval);
     }