BBC2-41 add GitHub Actions workflow to run bun test on PRs

[b2l]

Summary

• Adds .github/workflows/ci.yml.
• Triggers: every PR (opened/synchronized/reopened) and every push to main.
• Steps: checkout → setup-bun@v2 pinned to 1.3.11 → bun install --frozen-lockfile → bunx tsc --noEmit → bun test.
• concurrency cancels superseded runs on the same ref so rapid pushes don't queue duplicate jobs.
• Typecheck runs before tests: bun test doesn't catch type errors on its own; putting tsc first fails fast.

Manual follow-up

Requiring CI to pass before merge is a GitHub branch-protection setting, not something this PR can enable. After landing, configure at https://github.com/b2l/bbcli/settings/branches → add a rule for main → require the test check.

Test plan

• bunx tsc --noEmit — clean
• bun test — 128 pass
• This PR's own CI run is green (validates the workflow end-to-end)