[codex] Clarify source checkout install flow

[Erwin2233]

What changed

This updates the installation documentation in both English and Chinese README files.

• Clarifies the published package install path.
• Updates the pnpm requirement from 9+ to 10+.
• Adds a source-checkout run path for developers who are working without a prebuilt dist/ tree.
• Notes that team control-plane commands in the public CLI are still stabilizing.

Why

A source checkout can be confused with a packaged release because ./velaclaw.mjs expects built output. The README now gives explicit development commands so contributors can run the CLI reliably from source.

Validation

• git diff --check -- README.md README.zh-CN.md
• pre-commit hook detected docs-only changes and skipped pnpm checks