docs: extend agent auth code validity from 5 to 10 minutes

[hold-baby]

Merge Verdict

[APPROVE] — Docs-only copy change; auth code TTL updated from 5 to 10 minutes, consistently applied across all three locales.

3 files · +9 / -9 · docs/en docs/zh-CN docs/zh-HK

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Change Summary

• Updates the AI Agent auth code TTL from 5 minutes to 10 minutes on the agent-auth page.
• All 3 mentions per file are covered: the intro statement, the Security bullet (10-minute TTL), and the "code has expired" troubleshooting note.
• Applied identically in en / zh-CN / zh-HK, satisfying the repo's trilingual sync rule.

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Risk Analysis

Risk	Level	Mitigation
Docs drift from backend TTL	🟡	Requires the server-side auth code TTL to actually be 10 minutes; otherwise users will hit an expiry earlier than documented
Missed occurrences	✅	grep over docs/ confirms no remaining "5 minutes / 5 分钟 / 5 分鐘" reference to the auth code TTL
Locale divergence	✅	Same 3 edits in all three locale files, verified line by line

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Design Decisions

• Terminology kept intact: only the number changed; wording, bold markers and links are untouched, so translation parity is preserved.
• Device-flow polling left alone: docs/{lang}/skill/install/index.md mentions "~5 minutes" for the auth login device-code polling window — a different mechanism from the one-time auth code, correctly not modified.

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Code Concerns

1. [需判断] Docs change assumes the backend TTL is already 10 minutes (docs/en/docs/agent-auth.md:37)
   Docs are not the source of truth here; confirm the auth service has shipped the 10-minute TTL before merging, otherwise the page over-promises.

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Verification Status

✅ Markdown-only change · ✅ Trilingual sync verified · 📋 To verify: backend auth code TTL is actually 10 minutes