chore: migrate from pnpm to Bun 1.3.2

- Update package manager from pnpm@9.15.0 to bun@1.3.2
- Rewrite Dockerfile to use Bun official image (oven/bun:1.3.2-slim)
- Update all GitHub Actions workflows to use oven-sh/setup-bun@v1
- Update all documentation (CLAUDE.md, AGENTS.md, CONTRIBUTING.md, README)
- Replace pnpm-lock.yaml with bun.lock
- Update .gitignore and .prettierignore for Bun debug logs
- Remove pnpm from CI/CD allowed tools list

Changes verified:
- ✅ bun run typecheck: passed
- ✅ bun run lint: passed (only pre-existing warnings)
- ✅ bun run build: successful
- ✅ Docker build: successful (437MB image)
- ✅ Docker runtime: verified