docs: rebuild EN/ZH READMEs based on current implementation

[AstroMen]

Motivation

• The repository README files contained mixed or stale fragments after previous merges, causing incomplete or inconsistent documentation relative to the current code.
• The docs need clear, language-separated English and Chinese READMEs that accurately reflect the present backend/frontend implementation and developer workflow.

Description

• Rewrote README.md into a complete English-only README aligned with the current code, covering repo structure, Tickets/Runs/Agents/Audit flows, auth/token behavior, quickstart, env vars, API smoke checks, tests, CI, and troubleshooting.
• Rewrote README.zh-CN.md into a complete Chinese-only README with parity to the English file and no cross-language content, including quickstart, env vars, API checks, tests, CI, and troubleshooting.
• Removed mixed-language and stale fragments from both files and clarified notes about NEXT_PUBLIC_* token alignment, ./scripts/start_dev.sh behavior, and how to run API smoke checks and tests.
• Updated in-file examples and references to docs/INSTALL.md and docs/RBAC.md to point to current documentation.

Testing

• Ran an automated Python check that asserts language separation by verifying README.md contains no CJK characters and README.zh-CN.md contains CJK characters, and the check succeeded.

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Codex Task