fix: correct broken links, stale versions, and dead references in docs

- README: fix LICENSE link, remove dead doc links, update install
  instructions, remove hardcoded version/bundle size/LOC metrics
- artifacthub-pkg.yml: fix appVersion 0.2.18 → 0.2.20, fix README
  path, fix build-from-source cd path
- docs/README.md: trim to only reference files that actually exist
  (was 160 lines of aspirational links, now 47 lines of real ones)
- CLAUDE.md: correct "no MUI imports" claim — code uses @mui/material
- headlamp-plugin-developer agent: match corrected MUI convention

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -68,7 +68,7 @@ Uses custom hooks (`hooks/`) and a utility library (`lib/`) instead of a single
 
 - Functional React components only — no class components
 - All imports from `@kinvolk/headlamp-plugin/lib` and `@kinvolk/headlamp-plugin/lib/CommonComponents`
-- No additional UI libraries (no MUI direct imports, no Ant Design, etc.)
+- MUI (`@mui/material`) is available via Headlamp's bundled dependencies — no other UI libraries (no Ant Design, etc.)
 - TypeScript strict mode — no `any`, use `unknown` + type guards at API boundaries
 - Tests: vitest + @testing-library/react, mock with `vi.mock('@kinvolk/headlamp-plugin/lib', ...)`
 - `vitest.setup.ts` provides a spec-compliant `localStorage` shim for Node 22+ compatibility