docs: refresh README and usage guides

[YoungJinJung]

Summary

• update README to match the current CLI flow (key share, key use, register, encrypt, decrypt, doctor)
• refresh USAGE_EN.md and USAGE_KO.md so the sender/receiver workflow matches the latest commands and defaults
• document current inspection and safety options such as recipient show, sender show, --confirm, --no-clobber, and --out-temp

Notes

• docs-only change
• no code behavior changed

[amazon-q-developer]

This documentation update correctly reflects the current CLI command structure and workflow. All changes consistently update command names (send→encrypt, receive→decrypt) and add documentation for new features (key share, key use, register, doctor) along with safety options (--confirm, --no-clobber, --out-temp). The updates are well-structured and maintain consistency across README.md, USAGE_EN.md, and USAGE_KO.md. No blocking defects found.

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