Add CLI integration tests for config precedence and exit codes

[LyraZeta]

Why

LayoutScope v0.3.0 supports project config discovery and explicit CLI overrides, but the current coverage does not exercise the full precedence and exit-code contract from the command-line process boundary.

Scope

Add focused, cross-platform integration tests that launch the CLI against a deterministic local fixture and cover:

• explicit --path, --viewport, --wait, --ready, and --fail-on values overriding their config-file counterparts
• --no-auto-config ignoring a discovered layoutscope.config.* file
• an explicit --config still loading when auto-discovery is disabled
• exit code 0 for a passing scan or --fail-on never, 1 when the selected threshold is reached, and 2 for invalid configuration or CLI input

Acceptance criteria

• Tests use a local fixture and make no external network requests.
• Assertions prefer JSON/report metadata over ANSI-formatted terminal text.
• The test setup works on Windows and Linux without shell-specific command strings.
• pnpm check passes.
• Production behavior stays unchanged unless the integration test exposes a real bug, in which case keep the fix narrowly scoped and explain it in the PR.

This is deliberately test-focused; using the existing fixtures and helpers is encouraged.