fix(ci): repin mirror.yml to a reachable standards SHA (892497fe is unloadable)

[hyperpolymath]

Repins mirror.yml from 892497fe to a SHA Actions can actually load.

Measured cause

mirror.yml is pinned to a standards SHA that Actions refuses to load. GitHub's own
annotation on the failed run, anchored at .github/workflows/mirror.yml#L15:

error parsing called workflow ".github/workflows/mirror.yml"
-> "hyperpolymath/standards/.github/workflows/mirror-reusable.yml@<SHA>"
: workflow was not found.

The run dies at startup: jobs.total_count == 0, no logs, no annotations in REST. This repo
has therefore mirrored to zero forges since the pin landed.

Why three separate probes certified the broken pin as good

probe	result on the bad pin
repos/standards/commits/$SHA resolves	PASSES
contents/mirror-reusable.yml?ref=$SHA serves the file	PASSES
actionlint on the fetched body	PASSES (0 errors)

The discriminator is reachability, not existence:
gh api repos/hyperpolymath/standards/compare/main...$SHA --jq .status.
behind/identical = reachable from a ref. ahead/diverged = not, and Actions refuses it.
Measured across 8 pins: the 6 used by every working repo are behind; the 2 that are
diverged account for every startup death in this class, 3 repos, 3 of 3, zero false positives.

The estate gate .claude/scripts/check-phantom-pins.sh has been repaired to carry this second
probe and exercised against both a bad and a good fixture.

Target

da2c748aad55c1a1dcba00b60fe4a35017bc6540 — reachable (behind main by 76), and already the
target of the in-flight pin-bump campaign (natsci-studio#82), so this converges rather than
creating a new cohort.

Verified contract-identical to the pin used by the working repos: same 7 forge jobs
(gitlab, bitbucket, codeberg, sourcehut, disroot, gitea, radicle), zero required inputs,
zero required secrets, zero tag-pinned actions. No forge is lost and no new secret is
needed.

Scope

One line, one file. No permissions: block is touched.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between a5c533d and 1a61276.

📒 Files selected for processing (1)

• .github/workflows/mirror.yml

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