Nothing gates the CI-repro build command against `ci.yml` — three docs now duplicate it

[azchohfi]

Summary

#1165 fixed #1163 by correcting the pre-push build command, and in doing so brought a third copy of it into the tree. The CI-repro command now appears verbatim in three places:

File	Audience	Depth
.github/PULL_REQUEST_TEMPLATE.md	outside contributor	one line + short rationale
CONTRIBUTING.md (Reproducing the CI build gate)	contributor deep-dive	full explanation
AGENTS.md (Field notes → Building & running tests)	agents	full + measurement

The layering is deliberate — different audiences need different depth, and the template defers to CONTRIBUTING.md. The problem is that nothing gates the three against ci.yml or against each other.

Why this is worth a gate rather than a comment

This is the exact failure mode that produced #1163. That issue existed because AGENTS.md and .github/PULL_REQUEST_TEMPLATE.md disagreed about the same command, and the disagreement went unnoticed because no check compared them. #1165 resolved the disagreement by hand — but by adding a third copy, it made the same drift structurally more likely next time, not less.

Concretely, if the build-solution job in .github/workflows/ci.yml ever changes (a new flag, a different target, a split step), all three docs silently become wrong, and the first signal is a contributor pushing a PR that fails a gate their documented command said they had already cleared.

Line-number drift already bit this once: #1163 cited ci.yml:342 for a step that had moved to :329 before the issue was filed. #1165 replaced line references with job/step-name anchors, which fixes that axis — but not the command-text axis.

Suggested fix

A unit test in tests/Reactor.Tests/Docs/ that parses the Restore and Build steps of the build-solution job out of .github/workflows/ci.yml, and asserts the documented command in each of the three files agrees with them.

There is direct precedent for reading repo files from a unit test: tests/Reactor.Tests/Docs/SwallowedErrorAuditTests.cs:33 does exactly this (const string AuditPath = "docs/specs/044/swallowed-error-audit.md"), and the repo already runs several drift gates in this style — the swallowed-error ledger, the reactor.api.txt two-copy sync, and the ReactorGallery search-index byte-compare.

The oracle should be non-vacuous: it must fail if the CI step changes and the docs don't, so the test needs to derive the expected string from ci.yml rather than hard-coding it alongside the docs. A hard-coded expectation would restate the docs and pass forever.

Notes

• Low priority — the three copies agree today, verified byte-identical on main as of c9225296.
• Worth considering whether the template's copy should shrink to a pointer at CONTRIBUTING.md instead, reducing three copies to two. That trades a little contributor convenience (the command is no longer inline where it's read) for one less drift surface; the gate is probably the better fix either way.
• Filed as a follow-up from the PR template's suggested build command can't reproduce the Build solution CI job (missing -c Release) #1163 / Docs: make the documented pre-push build actually reproduce the Build solution CI job #1165 work rather than folded in, since it's a testing change and Docs: make the documented pre-push build actually reproduce the Build solution CI job #1165 was deliberately markdown-only.