Fix GitHub auth lint failures and harden CI setup

[skulidropek]

Summary

• replace the GitHub user decode path with Schema.parseJson(...) so token validation no longer uses a forbidden unknown boundary type
• refactor GitHub auth status aggregation to avoid callback-reference lint violations and reduce nested inline logic
• keep the status report behavior unchanged while making the lib lint step pass again
• update the shared GitHub Actions setup to use pnpm/action-setup@v5
• switch the shared dependency install step to pnpm install --frozen-lockfile so Check and dependency validation run with the same install mode

Verification

• pnpm install --frozen-lockfile
• pnpm --filter ./packages/app check
• pnpm --filter ./packages/lib typecheck
• pnpm --filter ./packages/lib exec eslint src/usecases/auth-github.ts src/usecases/github-token-validation.ts --ext .ts