docs(plans): widen Step 20a's Files list to the two npm doc repairs

What this delivers

Merge request !2162 (merged) implements Step 20a and touches two paths that the step's Files entry does not name. This merge request adds both paths to that entry, and corrects one bullet in the same entry that named half of the sections the step writes. Guardrail 4 in AGENTS.md keeps a step merge request out of the plan file, so the record lands here.

The two paths

• internal/datastore/lifecycle_reap_npm.go came in through a comment-caps repair, not through new behavior. The docs on deleteNpmAttachmentHolderBatch and reapNpmTags carry a claim that the step's new two-leg caller makes false. Each doc compresses to the one line scripts/ci/check-comment-caps.sh allows an unexported top-level doc. The corrected reasoning moves to docs/dev/storage-accounting.md, under ### Hosted npm reapers.
• internal/datastore/lifecycle_reap_npm.md is outside the plan's original scope for this step. One clause cites the same symbol's doc for a statement that the compression removes. That clause points at docs/dev/storage-accounting.md, under ### Hosted npm reapers, instead.

Neither path adds behavior. Both are comment and document corrections that guardrail 17 in AGENTS.md compels.

The corrected section list

The entry's docs/dev/storage-accounting.md bullet named two sections. The step also writes ### Hosted npm reapers, which takes the reasoning compressed out of deleteNpmAttachmentHolderBatch, releaseNpmAttachments and reapNpmTags. The bullet now names all three, and says they are the whole of what the step writes in that file. A half list is worse than none, because a reader takes the sections it does not name as untouched. On the step branch, git diff beb8ad201e..HEAD -- docs/dev/storage-accounting.md returns three hunks and no fourth section.

Scope

The diff is 13 added lines and 1 removed line in one file. No other step's entry changes, and this merge request does not touch the Status table. No end-to-end scenario changes, because a plan file carries no behavior.

Related to #728

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