Implement a --comment option for sourcemeta-one-index

[jviotti]

Signed-off-by: Juan Cruz Viotti jv@jviotti.com

[augmentcode]

🤖 Augment PR Summary

Summary: Adds a new CLI option to persist an informational comment alongside generated index output.

Changes:

• Introduced a --comment / -m option to sourcemeta-one-index.
• When provided, writes the comment to comment.json in the staging/output directory (via write_json_if_different).
• Adjusted the step numbering in index_main to account for the new “store comment” stage.
• Added CLI regression tests to ensure comment.json is removed when the option is omitted on rebuild, and updated when changed on rebuild.

Technical Notes: The new artifact participates in the existing staging+hardlink workflow, so incremental rebuilds can update or delete comment.json deterministically via the output tracking/removal pass.

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[augmentcode]

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[cubic-dev-ai]

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