#6235: fix: from source development command

Since pnpm build only compiles the project files into the dist/ directory but does not install the openclaw binary to your system's global PATH, the command openclaw explicitly will not be found in your terminal. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the “From source (development)” instructions to run onboarding via `pnpm openclaw ...` instead of relying on a globally installed `openclaw` binary, aligning the docs with the fact that `pnpm build` produces `dist/` output but doesn’t put `openclaw` on PATH. Change is localized to `docs/start/getting-started.md` and improves the out-of-the-box developer experience when building/running OpenClaw directly from the repo. <h3>Confidence Score: 4/5</h3> - This PR is safe to merge and only adjusts documentation guidance. - The change is a small docs-only update that makes source-development instructions more accurate; the only risk is minor user confusion if adjacent examples use different invocation styles. - docs/start/getting-started.md <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->