build: update pnpm to v12

[dangreen]

Why

pnpm 12 is the latest release, one major past the 11.9.0 the repository was on. 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 lockfile stays as it is: since #195 it records linkDirectory: false for the 8 packages whose publishConfig sets it, which pnpm 12, like pnpm 11.28, checks in a frozen install.

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.

Notes

• Stacked on build: update Node.js to v26 and pnpm to v12 #198, which pins Node.js 26 in the same .tool-versions. Once build: update Node.js to v26 and pnpm to v12 #198 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.
• Checked on pnpm 12.9.1 and Node.js 26.10.0: a frozen install from scratch, an install that leaves the lockfile alone, pnpm run lint:package-json, which triggers no install before the script, and the unit tests and oxlint of oxlint-config.
• The Renovate PRs chore(deps): update dependency pnpm to v12 - autoclosed #196 and chore(deps): update dependency pnpm to v11.28.5 - autoclosed #183, which bump pnpm, become obsolete.

[dangreen]

Folded into #198, which now carries the pnpm 12 update as well.