docs: clarify admin_from for privileged commands

[xukp20]

Summary

Clarify how admin_from should be configured for privileged commands such as /dir.

This documents the configuration detail discussed in #359: admin_from must be defined at the [[projects]] level, not under
[projects.platforms.options].

What changed

• clarify admin_from placement in config.example.toml
• document that /dir is a privileged command requiring admin_from
• explain how to obtain the user ID via /whoami or /status
• document the admin_from = "*" shortcut and its risk in multi-user environments
• add the same clarification to both English and Chinese usage docs
• add a short note in README.md

Why

A user in #359 was able to fix the problem after moving admin_from to the correct config level, and suggested documenting it.

This PR makes that guidance explicit so users do not accidentally place admin_from under platform options and wonder why privileged commands are still blocked.

Validation

• reviewed latest upstream/main before editing
• go test ./config ./core ./agent/... ./platform/...
• manual diff review of all doc changes

Note:

• full go build ./... / go test ./... on a clean latest main checkout is currently blocked by missing web/dist in the repository checkout, which
  is unrelated to this doc-only change

Related issue

• Related to [Bug] 飞书中/dir系列命令无效 #359

[chenhg5]

LGTM. Helpful documentation clarification.

Review summary:

• ✅ Clarifies admin_from placement (project level, not platform options)
• ✅ Documents how to get User ID via /whoami / /status
• ✅ Bilingual docs updated
• ✅ CI passes

Addresses the confusion from #359.