CI: docs archive checks red on develop after #1395 + #1420 — prune inventory names a non-candidate file on the push event

[radandevist]

Develop's push CI is red on docs archive checks since f7d393b4e (run 32859795683), while every PR involved was green at its tip.

Failing step: "Check prune inventory freshness (--check)":

Error: Decision table names a non-candidate file: docs/archive/2026/designs/2026-07-09-front-2-gray-ui-stack-migration-design.md

Context: #1395 (merged as 86f3acbb4) pruned docs/ and introduced the prune inventory + git-fidelity gate; #1420 (merged as f7d393b4e, P1 canary fix) only touched docs/deployment/production-deploy-runbook.md and API code. The PR checks ran on the PR merge commits and passed; the same check run on the develop push event fails — so the generator's notion of "candidate files" depends on the event/ref it runs on (probably the diff range or the set of files present at a different ref), not on the tree alone.

Required:

1. Reproduce locally on f7d393b4e with the exact CI command; state the root cause in plain words in the PR body (why a green PR check turns red on the push event).
2. Fix so the check is a pure function of the committed tree (same result on PR and push events), or make the decision table consistent with the pruned tree — whichever is correct-by-design; never loosen the guard or delete the failing row without explaining why the row was wrong.
3. Paired proof: the failing state RED locally, GREEN after the fix; the PR's own CI AND a note on how develop's push run will be green after merge.

This is the "two green PRs can turn develop red" class; PR CI never tests PR+PR. Follow-up to #1395.