chore: use a classic .gitignore

[YosefHayim]

What changed

• .gitignore rewritten in the classic grouped style: dependencies, build, logs, editor and OS. Added Thumbs.db, .idea/, .vscode/, *.swp because there is no global ignore file.
• Removed 8 ADR citations from code comments and two schema description strings (src/commands/login.ts, vendor/shared/billing/contracts.ts, vendor/shared/billing/pricing.ts). The explanations stay; only the ADR-00xx ids are gone. No behavior change.

No root docs/ folder exists, so nothing was deleted.

Note: vendor/shared is synced from the genshot monorepo; the same strip should land there so the next sync does not bring the ids back.

How it was checked

• pnpm install --no-frozen-lockfile, pnpm typecheck, pnpm test (14 passed), pnpm build.
• git grep -n -i -w adr prints nothing.
• No tracked file becomes ignored (git ls-files -ci --exclude-standard is empty); the local checkout has no ignored files that would become untracked.