fix(agent): handle UTF-16 files in agent edit

[hechibing]

Summary

• replace UTF-8-only reads in �gent edit flow with
  ead_file_with_encoding
• use byte-for-byte backup for rollback so original encoding is preserved
• add regression test covering UTF-16 LE + BOM read path used by edit flow

Why

�gent edit used
ead_to_string, which fails on UTF-16 files even though agent loading supports UTF-16.

Validation

• cargo test -p cortex-cli test_read_file_with_encoding_supports_utf16_for_edit_flow -- --nocapture

Related bounty report: PlatformNetwork/bounty-challenge#11203

[greptile-apps]

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