Fix usage scanner fixture false positives

[MANOJ-80]

Closes #861

Summary

• Exclude example/test/fixture directories from usage scanning.
• Prevent PD001/PD002 from treating fixture/demo imports as root project source usage.
• Add regression coverage at scanner and override context levels.

Verification

npm test -- tests/usage.test.ts tests/overrides/context-builder.test.ts tests/overrides/detectors/pd001.test.ts tests/overrides/detectors/pd002.test.ts --runInBand
npm run build

[Copilot]

Pull request overview

This PR addresses false-positive phantom-dependency findings (notably PD001/PD002 under --check-overrides) by ensuring the usage scanner ignores imports that appear only in example/test/fixture directories, and adds regression tests to lock the behavior in.

Changes:

• Add a usage-scanner–specific exclusion set that extends the shared EXCLUDED_DIRS with examples/, tests/, fixtures/, __tests__/, and __fixtures__/ (plus singular forms).
• Update both scanProjectForPackageUsage and scanAllImports to apply the new usage-specific directory exclusions.
• Add regression tests covering scanner behavior and buildOverrideContext integration to ensure excluded directories don’t influence importedPackageNames.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/usage/scanner.ts	Introduces USAGE_EXCLUDED_DIRS and applies it to directory traversal for both usage-scan entrypoints.
tests/usage.test.ts	Adds regression tests verifying example/test/fixture directories are excluded for both targeted and full import scans.
tests/overrides/context-builder.test.ts	Adds integration coverage ensuring override context import collection ignores excluded directories.

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