Summary:

- Stop loading persisted default Zed Rules into native agent project
context.
- Remove legacy user-rules rendering from agent system prompt templates.
- Keep Skills and project rules file support unchanged.

Tests:

- cargo test -p agent
test_system_prompt_does_not_render_legacy_zed_rules_section --lib
- cargo test -p prompt_store test_empty_skills_sets_has_skills_false
- cargo test -p prompt_store
test_project_context_does_not_filter_by_budget

Closes AI-325
Release Notes:

- Fixed deprecated Rules being automatically included in new agent
requests.