build: update pnpm to v12

[dangreen]

Why

pnpm 12 is the latest release, one major past the 11.9.0 the repository was on, while CI ran 11.11.0. Its breaking changes leave the workspace alone: pnpm 12 recognizes every setting of pnpm-workspace.yaml, nothing passes --frozen-lockfile false, no dependency comes from git, and engineStrict is off. The one change it asks for is in the lockfile: pnpm 12, like pnpm 11.28, compares the publishConfig.linkDirectory of a package with the lockfile, so a frozen install of the old lockfile fails with ERR_PNPM_OUTDATED_LOCKFILE.

What

• .tool-versions pins pnpm 12.9.1, the version of the latest tag.
• Every pnpm/action-setup step asks for pnpm 12; v6 of the action supports it.
• The lockfile records linkDirectory: false for the 8 packages whose publishConfig sets it, the way pnpm 12 writes it, and nothing else. pnpm 12 finds it up to date with these lines alone, and a plain install changes nothing.

Notes

• Stacked on build: update Node.js to v26 and pnpm to v12 #203, which pins Node.js 26 in the same .tool-versions. Once build: update Node.js to v26 and pnpm to v12 #203 is merged, this branch gets rebased onto main, which also runs the Checks workflow that covers pull requests into main alone.
• minimumReleaseAge has applied since pnpm 11, so the move to 12 changes nothing about it, and minimumReleaseAgeExclude stays as it is.
• Checked on pnpm 12.9.1 and Node.js 26.10.0: a frozen install from scratch, a plain install that leaves the lockfile alone, the build of core through pnpm run, which triggers no install before the script, the unit tests and lint of core, and lint-package-json through pnpm dlx.
• The Renovate PRs chore(deps): update dependency pnpm to v12.10.1 #200 and chore(deps): update dependency pnpm to v11.28.5 - autoclosed #162, which bump pnpm, become obsolete.

[coveralls]

Coverage Report for CI Build 37490810327

Warning

No base build found for commit 373adb4 on build/node-26.
Coverage changes can't be calculated without a base build.
If a base build is processing, this comment will update automatically when it completes.

Coverage: 69.769%

Details

• Patch coverage: No coverable lines changed in this PR.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

Requires a base build to compare against. How to fix this →

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Coverage Stats

Relevant Lines:	1046
Covered Lines:	725
Line Coverage:	69.31%
Relevant Branches:	211
Covered Branches:	152
Branch Coverage:	72.04%
Branches in Coverage %:	Yes
Coverage Strength:	10.93 hits per line

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💛 - Coveralls

[dangreen]

Folded into #203, which now carries the pnpm 12 update as well.