chore(tooling): migrate workspace config to pnpm 11

[kubohiroya]

Summary

• Pin the repository package manager to pnpm 11.25.0.
• Raise the supported runtime to Node.js >=22 and constrain pnpm to the 11.x line.
• Move overrides and patched dependencies from package.json#pnpm to pnpm-workspace.yaml, which is the pnpm 11 configuration source.
• Replace removed ignoredBuiltDependencies configuration with explicit allowBuilds decisions, including core-js discovered by pnpm 11 strict build validation.
• Document the pnpm 11 and Node.js runtime contract.

Verification

• pnpm --version — 11.25.0
• pnpm install --frozen-lockfile — exit 0
• pnpm run ci:checks — exit 0
• pnpm typecheck — exit 0, 238/238 tasks successful
• pnpm test — exit 0, 62/62 tasks successful
• pnpm build — exit 0, 129/129 tasks successful
• pnpm lint — exit 0, 14/14 tasks successful
• git diff --check — exit 0
• pnpm config get confirms overrides, allowBuilds, and patchedDependencies are loaded from pnpm-workspace.yaml.

Rollback

• Revert commit 4f63779 to restore pnpm 10.29.3, the previous engine constraints, package.json pnpm configuration, and the prior workspace configuration.

Closes #1742

[kubohiroya]

Self-review: checked the pnpm 11 migration diff for scope and configuration placement. The change is limited to root package manager/engine metadata, pnpm workspace settings, and matching docs. The lockfile-only install reports the lockfile is already up to date under pnpm 11.25.0, and shims:check plus git diff --check passed. No code-path regression found in this diff. Remaining risk is CI coverage for the repository package-manager bootstrap on GitHub-hosted runners.