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/session-cli-builder.ts

@@ -9,7 +9,7 @@
  */
 
 import type { ClaudeMode } from './types.js';
-import { getAugmentedPath } from './utils/claude-cli-resolver.js';
+import { getAugmentedPath } from './utils/index.js';
 
 /**
  * Build Claude CLI permission flags based on the configured mode.