Implement three-scope configuration resolution (local > project > global)
with deep merge. Add ConfigScope enum, project-root discovery, and
config.local.toml file support.
Changes:
- Add LOCAL to ConfigLevel enum and new ConfigScope enum (GLOBAL, PROJECT, LOCAL)
- Implement discover_project_root() walking up from CWD for cleveragents.toml/.cleveragents
- Implement config.local.toml file loading in ConfigService
- Implement three-scope deep merge via _deep_merge() helper
- Add read_project_config(), read_local_config(), read_merged_config() methods
- Add write_scoped_config() for writing to any scope file
- Update set_value() to accept optional scope parameter
- Update resolve() with six-level precedence chain (CLI > env > local > project > global > default)
- Update CLI config set with --scope flag (global/project/local)
- Add config.local.toml to .gitignore and DEFAULT_IGNORE_PATTERNS
- Add Behave BDD scenarios for three-scope resolution, deep merge, and project-root discovery
ISSUES CLOSED: #937
