Scanner: extend IGNORED_FILES with HISTORY.md, CHANGES.md, AUTHORS, NOTICE

[Railly]

Context

src/scanner.ts ignores readme.md, license, changelog.md, .ds_store, thumbs.db, but skill folders in the wild also ship HISTORY.md, CHANGES.md, AUTHORS, NOTICE, .gitignore, .gitkeep, which currently pollute the Browse list as if they were skill files.

Task

Extend the ignore set and cover it with a scanner test.

Acceptance criteria

Test confirms these filenames are excluded from scan results.