Honor scan exclusions in external tool findings

[benjaminsehl]

Problem

Generic external tools discover their own files, so configured Desloppify exclusions could be ignored by their diagnostics. For example, scanning a nested Swift project with ios/.build excluded still reported SwiftLint violations from the generated SwiftPM test runner.

Change

Filter parsed tool findings through the existing project-relative path resolver and exclusion matcher. Validate the original tool output first so an exit code indicating lint findings still succeeds when all diagnostics are excluded. Preserve parser metadata and retained findings.

Validation

• Ten new regression cases failed before the fix and pass afterward. Coverage includes absolute and nested relative paths, component/glob exclusions, similarly named siblings, all-excluded nonzero output, direct detectors, and NextLint paths already normalized to the project root.
• Independently reran the exclusion, generic-plugin, and NextLint tests: 81 passed. The broader focused run passed 99 tests; Ruff and diff checks passed.
• Real SwiftLint output: 976 parsed findings became 954; all 22 generated-file findings were removed and every other finding was preserved.
• Full suite: 6,681 passed, 165 skipped, 3 Bash unused-import tests failed. Those same failures reproduce on the unmodified upstream commit 3a7735d; the installed Python 3.14 tree-sitter-language-pack distribution lacks its importable module in this environment. Keeping this draft to make that validation limitation explicit.