📖 docs: add missing build step to quick start

[tenlisboa]

Summary

The README's Quick Start skips straight from bun install to bun start, but bun start resolves to bun dist/index.js (via src/cli/package.json) and dist/ does not exist after a fresh install. A first-time follower of the docs hits:

$ bun start
$ bun run --cwd src/cli start
$ bun dist/index.js
error: Module not found "dist/index.js"
error: script "start" exited with code 1

This PR inserts a Build step between Install and Run so the documented flow actually works end to end.

Reproduction

git clone … && cd tinyclaw
bun install
bun start        # <-- fails: Module not found "dist/index.js"

After the fix:

bun install
bun run build    # <-- added
bun start        # <-- CLI runs and prints usage

Notes for reviewer

• Targeted dev per CONTRIBUTING.md.
• Minimal, docs-only change — no code touched.
• Separately, bun start alone prints CLI usage rather than opening http://localhost:3000; reaching the web UI seems to need tinyclaw setup --web / tinyclaw start. Happy to follow up in a second PR if you confirm the intended first-run flow.

Checklist

• PR targets dev
• Commit follows Clean Commit (📖 docs: …)
• Docs-only — no build/test impact

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}