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/config/terminal-limits.ts

@@ -11,11 +11,5 @@
 /** Max input length per API request (bytes) */
 export const MAX_INPUT_LENGTH = 64 * 1024;
 
-/** Max terminal columns for resize requests */
-export const MAX_TERMINAL_COLS = 500;
-
-/** Max terminal rows for resize requests */
-export const MAX_TERMINAL_ROWS = 200;
-
 /** Max session name length (chars) */
 export const MAX_SESSION_NAME_LENGTH = 128;