Clean build: remove all dead code and fix warnings
- Remove unused fields (trigger_offset on Requesting, is_incomplete)
- Remove unused methods (selected_item, has_actions, is_menu_open,
close_menu, move_selection, confirm_code_action, apply_workspace_edit)
- Remove unused import (Shrinkable in signature_help)
- Remove all #[allow(dead_code)] annotations
- Add build standards to AGENTS.md: zero warnings, zero errors required
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>