Fix vacuous color assertions in TestSummarizeChecks

[joshuacrass]

Problem:
In main_test.go, TestSummarizeChecks asserts against expressions like colorGreen + "✓" + colorReset. However, colorGreen and colorReset are defined as empty strings ("") in render.go as legacy compatibility constants. The test therefore asserts that summarizeChecks returns "✓" — which is true whether or not any colour is applied. The test would pass even if summarizeChecks returned a plain uncoloured string, making it useless for verifying that ANSI colour codes are correctly applied.

Solution:
Update the test to assert against the actual styled output using lipgloss style rendering, or — if colour is not the concern — assert only against the text content and document why colour is excluded from the assertion.

Acceptance Criteria:

• TestSummarizeChecks asserts against the actual styled output, not against empty-string colour constants
• The test fails if summarizeChecks stops applying colour to its output
• go test ./... passes