[codex] Add multi-author commit attribution

[explicit09]

Summary

• Adds a canonical authors: Vec<Author> commit field while keeping the existing primary author field for compatibility.
• Deserializes legacy single-author commits into a one-entry authors list.
• Adds multi-author commit APIs and wires CLI commit, watch, and merge paths to record co-authors from VEDIT_CO_AUTHORS entries formatted as Name <email> separated by semicolons.
• Surfaces all authors in vedit log and vedit show.

Validation

• cargo test
• Manual CLI smoke test with VEDIT_AUTHOR_NAME, VEDIT_AUTHOR_EMAIL, and two VEDIT_CO_AUTHORS entries, verifying log and show output.

Notes

This implements the valid core/CLI portion of #20 in the current repository layout. The issue also mentions vedit_blame and old crates/core/src/vc paths, but this repo currently has no blame command or those paths, so that part remains out of scope until such a surface exists.

Closes #20.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve primary author during commit deserialization

When both author and authors are present, this logic discards author whenever authors is non-empty and then derives the primary author from authors[0]. That is a regression for mixed-format commit JSON (e.g., producers that treat authors as co-authors-only), because a commit that previously resolved to the explicit author will now silently change attribution to a different person. Deserialization should keep author authoritative (or validate/normalize consistency) instead of overwriting it from authors.

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