Restore calver package versions on 2026-10-rc

[vctrchu]

Background

Every commit on 2026-10-rc since #4622 fails the test job:

api_version "2026-10" does not match the version supported by @shopify/ui-extensions-tester ("2027-00")

Trace: #4612 shipped its (version-gated) breaking change with a major changeset. The release PR #4622 then bumped @shopify/ui-extensions and @shopify/ui-extensions-tester from 2026.10.0-rc.4 to 2027.0.0-rc.5. These packages are calver-keyed — the tester derives its supported API version from its own npm version (api-version.ts), so the major bump produced the nonexistent API version 2027-00, and validateApiVersion rejects every example pinned to api_version = "2026-10".

On an RC branch the major.minor of the package version is the API version; breaking changes must release as minor bumps within the branch's calendar version (#4612's change was already version-gated in its own docs: "removed for API versions 2026-10 and later").

Solution

• Downgrade the retained changeset to minor (pos-remove-action-api-from-action-render.md). Changesets pre-mode retains consumed changeset files and recomputes each release from initialVersions + the highest bump across all of them — with the file still major, the next Version Packages (rc) would recompute 2027.0.0-rc.6 and revert any version correction. Verified via changeset version dry-run: with minor, the next computed version is 2026.10.0-rc.5. The breaking-change warning stays in the changeset text and PR Remove Action API from POS cart line-item action render target #4612; only the "Major Changes" changelog heading is lost.

• Restore both packages to 2026.10.0-rc.5 (next unpublished version; 2027.0.0-rc.5 is already on npm and can stay as a dead tag — consider npm deprecate after merge). Note this was unsustainable regardless of intent: the branch's final release 2026.10.0 is semver-lower than the published 2027.0.0-rc.5, so the rc stream could never converge to its own stable release.

• Fix the cross-dep (ui-extensions-tester → ui-extensions@2026.10.0-rc.5) and retitle the CHANGELOG entries.

• Add a guard test asserting API_VERSION matches ^\d{4}-(01|04|07|10)$. Scope honestly stated: Version Packages (2026-10-rc) (rc) #4622's test job already failed on the existing example test and was merged and published anyway, so no test closes that override path. What this guard adds: it checks validity (real quarter) rather than consistency (toml == version), so the tempting wrong fix — bumping the example's api_version to 2027-00 — stays red instead of going green, it also catches minor-overflow versions (2026-11), and its comment states the calver rule so the failure isn't misread as a stale example.

• Document the rule in AGENTS.md (calver semantics, never-major, pre-mode changeset retention) so future contributors and coding agents don't repeat this.

Follow-ups this PR doesn't do (process, not code)

• Gate Deploy/publish on the test job — red CI published to npm this time.
• Optional: a changeset lint rejecting major bumps while .changeset/pre.json exists — that fires at the feature PR (earliest point), before the bad version exists in-tree.

No changeset: this PR is the version correction; the next Version Packages (rc) continues from 2026.10.0-rc.5.

🎩

• yarn test --ci ui-extensions-tester: 98 passed (includes the new guard test)
• Ran the CI-failing scenario (examples/testing/checkout-basic-testing-example: install + test) locally on this branch: 3 passed. The pre-fix failure is from CI logs, not reproduced locally.
• type-check / lint / test: green on this PR's CI (first green test job on the branch since Version Packages (2026-10-rc) (rc) #4622)

Note for after merge: the npm rc dist-tag currently points at the broken 2027.0.0-rc.5, so npm install @shopify/ui-extensions-tester@rc serves the bad version until the next Version Packages (rc) publishes 2026.10.0-rc.5 and moves the tag. If that gap matters, retag manually (npm dist-tag add @shopify/ui-extensions-tester@2026.10.0-rc.4 rc) or deprecate 2027.0.0-rc.5.