docs: clarify Remote-SSH usage scope - #103

Summary

• Documents the current Remote-SSH scope: each Extension Host reads one CODEX_HOME, and usage from separate hosts is not combined automatically.
• Clarifies that usage on an SSH host is visible only when the extension runs on that remote Extension Host.
• Records the privacy and correctness boundaries for cross-host data, including that raw conversation logs and credentials must never be transferred and allowance percentages must never be added across hosts.

Verification

• Documentation-only change; runtime behavior is unchanged.
• The Simplified Chinese and English roadmap copies describe the same scope and boundaries.
• Repository checks passed.

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