Introduces a new optional `defaultProject` field on Config (and
MinimalConfig for init output) that acts as a fallback when the
`-p`/`--project` CLI flag is omitted.
Domain-layer changes:
- Config.default_project: Option<String> with camelCase serde rename
- Config::load validates that defaultProject matches a configured
project path (exact or case-insensitive suffix match), returning
ConfigInvalid on mismatch
- Config::effective_project(cli_flag) -> Option<&str>: cascading
resolver that prefers the CLI flag, then the config default, then None
- MinimalConfig.default_project with skip_serializing_if for clean
JSON output when unset
Tests added:
- effective_project: CLI overrides default, falls back to default,
returns None when both absent
- Config::load: accepts valid defaultProject, rejects nonexistent,
accepts suffix match
- MinimalConfig: omits null defaultProject, includes when set
- Helper write_config_with_default_project for parameterized tests
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>