docs: add English README and preserve Chinese documentation

[paceaitian]

What changed

• replace the top-level README with a concise English landing page for international visitors
• preserve the complete existing Chinese guide as README.zh-CN.md
• add bidirectional language navigation
• document the PACE lifecycle, installation, enforcement boundaries, integrations, runtime scope, and verification workflow in English

Why

The repository previously presented its full documentation only in Chinese. Separating an English landing page from the complete Chinese technical guide makes the project easier to evaluate and adopt internationally without discarding the detailed documentation existing users rely on.

Impact

Documentation only. No plugin runtime, hook behavior, schema, command, or migration code changes.

Validation

• node tests/run-all.js — 8/8 suites passed; hooks-e2e 482/482
• Claude plugin manifest validation passed
• all relative Markdown link targets exist
• manifest facts verified: 9 hook event types, 4 skills, 5 commands, 1 agent
• Chinese README mechanically matches the former README.md after removing the new language selector
• git diff --check passed