Run `corepack use pnpm@next-12` (resolved to 12.3.4) on every directory
with a package.json: the repo root, the 11 module workspaces, and all
submodules. Every packageManager field now carries the same pinned
version and hash; the root and backend move off 11.20.0.
Fix the composable-test-suite workspace config (esbuild allowBuilds
placeholder left by pnpm 12) so its install passes, and add the missing
packageManager fields to frontend/packages/ui and mcp/packages/plugin,
since corepack only updates existing fields.
Document the canonical update procedure in
.serena/memories/workflow/updating-pnpm.md.
AI-assisted-by: omen-alpha
Andrey Antukh