docs(opencli): add env preflight for browser token

[ssdiwu]

Summary

This PR adds a small but important troubleshooting and preflight flow for browser-backed opencli usage in agent runtimes.

It teaches the skill to:

• check PLAYWRIGHT_MCP_EXTENSION_TOKEN before repeatedly retrying browser authorization
• stop the retry loop when the token is missing from the current process env
• ask the user to provide the token or confirm persistence
• branch remediation by runtime:
  • OpenClaw / gateway service: prefer ~/.openclaw/.env + openclaw gateway restart
  • Claude Code / direct terminal: persist in the shell env used to launch the agent
• verify the fix by re-checking env presence and running a small opencli ... -f json command

Why

A common failure mode is:

• opencli works in a normal terminal
• the same task inside an agent runtime keeps asking for browser authorization again

In OpenClaw specifically, this often happens because the gateway runs as a background service and does not automatically inherit the current shell environment.

Changes

• add Environment Preflight to SKILL.md
• add OpenClaw-specific remediation guidance
• add bilingual troubleshooting notes to README.md

Scope

This PR is intentionally documentation and skill-behavior only.
It does not auto-write secrets without user confirmation.