refactor: decouple tools from system prompt

The system prompt sections no longer change based on which tools are available.
All tool-specific instructions are now in the tool descriptions themselves.

- codebase_search: Added CRITICAL usage instructions about using it first
- write_to_file: Added guidance about preferring other editing tools
- search_files: Added regex pattern crafting guidance
- browser_action: Added web development usage guidance

- rules.ts: Removed getEditingInstructions(), all tool conditionals
- capabilities.ts: Removed all tool availability conditionals
- tool-use-guidelines.ts: Removed codeIndexManager conditional guidelines
- objective.ts: Removed all parameters and conditional instructions
- system.ts: Updated calls to match new simplified signatures

- Deleted mode-aware-sections.spec.ts (tests for removed behavior)
- Updated sections.spec.ts for new signatures
- Updated snapshot files

Author: daniel-lxs