Expose builder name output

[mkusaka]

Summary

• Add a name output to the action metadata.
• Set the name output to the selected builder name after the Blacksmith remote builder is configured.
• Also set the output when reusing an existing non-default builder or creating the local fallback builder.
• Document the new output in the README.

Fixes #103.

Verification

• git diff --check
• node --check dist/index.js
• node -e "JSON.parse(require(\"fs\").readFileSync(\"dist/index.js.map\",\"utf8\")); console.log(\"ok\")"
• prettier --check README.md action.yml src/main.ts src/main.test.ts

I could not run pnpm install, pnpm test, or pnpm run build locally because the private Buf generated package requires BUF_TOKEN; without it, pnpm install --frozen-lockfile fails with a 404 for @buf/blacksmith_vm-agent.bufbuild_es from https://buf.build/gen/npm/v1/.

---

^{Need help on this PR? Tag /codesmith with what you need. Autofix is disabled.}

---

^{Need help on this PR? Tag /codesmith with what you need. Autofix is disabled. (Staging)}