docs(plans): S04-A step 5's gate reading is the merging engineer's

What this changes

Amends Step 5 of docs/plans/2026-09-03-s04a-per-format-credential-columns.md. Its Acceptance asked for the per-environment leftover-plaintext readings "read from the current boot logs by the engineer opening this step's MR". This moves them to the engineer merging it.

Why the original ruling cannot be met

The opener physically cannot take the reading. A usable reading has to postdate the completion of that environment's rollout of the code-only change that stops Maven binding the interim pair, and that change is !2629 (merged), the part 1 MR Step 5 is stacked on. At the moment the step MR opens, no such boot has happened in any environment, so the line either does not exist yet or reports a count taken against a table pods are still writing the pair into.

So the ruling as written asked for a figure that can only be taken later. The one point at which it can be taken is immediately before the merge that makes the discard irreversible, which is also where it is most useful.

What else this records

Two consequences follow from the reassignment and belong in the plan rather than in the step MR, since the step MR is not the single writer of this file:

• The step MR stays Draft until its per-environment table is filled. No CI job can check that table and no reviewer can supply it.
• The no-rollback window runs to the Up applying in each environment, not to the step MR merging. The Up runs at pod boot, so a rollback between merge and rollout is inside the window, not after it. docs/dev/column-encryption.md already carried that condition in its open-ended form.

Why its own MR

Guardrail 4: a step MR does not edit the plan file. This is the docs(plans) change that carries the amendment, and it merges before !2630 (merged).

Stacked MRs

Part	MR	Scope	Target
plan amendment	this MR	Step 5's gate reading moves to the merging engineer	main
1/2	!2629 (merged)	stop binding the interim pair	main
2/2	!2630 (merged)	drop the columns	!2629 (merged)'s branch

Required order: this MR, then !2629 (merged), then !2630 (merged). This one and !2629 (merged) are independent and touch no common file, so only their order relative to !2630 (merged) matters.

Open merge requests this branch conflicts with

None. This branch changes one file, and no other open MR touches docs/plans/2026-09-03-s04a-per-format-credential-columns.md.

Tests

No test covers plan prose. markdownlint-cli2 passes on the file, and the longest line this MR adds is 78 characters, inside MD013's cap.

Related to #417 (closed)