fix(cargo): show clippy error details in compact output

[TechWizard9999]

Description

rtk cargo clippy could collapse output to counts/rules while hiding actionable error text in some failure modes. This PR preserves concise output but always includes concrete error lines.

Related Issue

Closes #602

Changes Made

• enhanced clippy filter to collect and show error detail lines in compact output
• added an Error details section (up to 5 entries, with overflow summary)
• added regression test to ensure error text is present when clippy reports an error
• added changelog entry under Unreleased bug fixes

Files Changed

• src/cargo_cmd.rs - clippy filter enhancement + regression test
• CHANGELOG.md - unreleased bug-fix note for Output filtering hides actual Clippy error messages, only shows summary #602

Testing

• cargo fmt --all --check
• rtk cargo clippy --all-targets (2 pre-existing warnings in untouched files)
• rtk cargo test
• rtk cargo test cargo_cmd::tests::test_filter_cargo_clippy_includes_error_details

Checklist

• Code follows project style guidelines
• Self-review completed
• Documentation updated
• No breaking changes introduced

cc @pszymkowiak for review