docs(plans): record step 4's three merge requests

Dzmitry (Dima) Meshcharakourequested to merge
dmeshcharakou/repository-column-writers-status-step-4 into main

Fills row 4 of the repository-column-writers Status table with the three merge requests step 4 opened as, in merge order: !2494 (merged), !2495 (merged), !2496 (merged).

Guardrail 4 keeps the table to a single writer, so this lands from its own branch rather than from any step MR. Docs-only, so no related-issue line is owed.

Related to #1223 (closed)

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