Consolidate duplicated logic into canonical shared functions:
- Extract load_config_with_cli_overrides() to utils.rs
- Was duplicated in lib.rs and accumulative.rs with subtle differences
- lib.rs version had Chrome diagnostics + provider validation
- accumulative.rs version was missing both
- Now all callers use the complete canonical implementation
- Extract combine_project_content() to project_files.rs
- Was duplicated inline in lib.rs and agent_mode.rs
- Simplified implementation using iterator flatten
- Added unit tests for all cases
This eliminates drift risk where the duplicated implementations
could diverge over time (accumulative.rs was already missing
Chrome diagnostics and provider validation).
Agent: fowler
3a0b656161