docs: add Polish (pl-pl) localization for app and CLI

[Azkel]

Summary

Adds Polish (pl-pl) localization for the GitHub Copilot App and Copilot CLI workshops, including locale landing pages, localization rules for translators/evaluators, and Starlight wiring so Polish appears in the language selector.

Type of change

• Workshop content (lesson Markdown, images)
• Site shell (website/ Astro + Starlight wrapper)
• Copilot configuration (.github/copilot-instructions.md, instructions, agents, skills)
• Repo housekeeping (CI, dependabot, README, license)
• Other:

Verification

• cd website && rm -rf dist && npm run build succeeds (target: 36 routes × 7 locales + 1 redirect = 253 built pages excluding 404; build reports 254 HTML files including 404, or note any intentional change)
• Lychee link check passes:
  mkdir -p /tmp/lychee-root && ln -sfn $PWD/website/dist /tmp/lychee-root/copilot-workshops && lychee --offline --no-progress --root-dir /tmp/lychee-root 'website/dist/**/*.html'
• External GitHub URLs that I changed have been clicked manually (lychee runs offline)

Notes for reviewers

• Scope: docs/pl-pl/app/** and docs/pl-pl/cli/** only (same harness coverage pattern as other locales). VS Code and Cloud agent English sources are unchanged.
• Locale wiring: registers pl-pl in website/astro.config.mjs and updates README / AUTHORING / PR-template / build-skill page-count invariants from 6 → 7 locales (253 / 254 HTML files).
• Rules: new .github/skills/localizations/rules/pl-pl.md documents terminology (e.g. Lekcja in app vs Ćwiczenie in CLI, skill not umiejętność, pull request kept in English, środowisko for harness).
• Voice: consistent second-person singular (ty) with masculine forms conventional in Polish technical docs (zainstalowałeś, Powinieneś); product UI chrome and learner paste prompts stay English.
• Verification notes: local build reported 254 HTML files; lychee offline reported 0 errors (16731 total / 13225 OK / 3506 excluded). All 14 external github.com URLs introduced in the Polish docs returned HTTP 200 (same targets as the English sources). https://github.com/copilot/agents redirects to login when unauthenticated, which matches the English lesson behavior.