Readability: sync/async cleanup, DRY, type safety, config fixes

[WolfieLeader]

🎯 Changes

Sync/async correctness

• scanForIntents and scanLibrary were sync internally but declared async — removed the unnecessary async/await throughout source and tests
• Fixed 2 broken tests that used .rejects.toThrow() on sync functions (these would silently pass since the throw happens before expect() is called)

DRY

• Deduplicated discoverSkills — removed copy from library-scanner.ts, now imports from scanner.ts
• Tests import findSkillFiles from src/utils.js instead of redefining it locally
• Extracted repeated test helpers: mockFetchVersion, mockFetchNotOk, requireFirstSkill, readPkgBin, readPkgFiles

Type safety

• Type narrowing guards instead of ! non-null assertions and as casts
• Removed redundant (obj[field] as string) casts where TS already narrowed the type

Config

• ESLint: allow T[] syntax, import order as warning, type-safe config export
• tsconfig: removed outDir/declaration (tsdown handles builds), added noEmit: true, included tests/ in type checking
• knip: added source entry points — knip can't trace dist/*.mjs back to source files, so intent-library.ts was falsely reported unused

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

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commit: 9888da8