Remove Yarn package manager support

[zkochan]

Proposed Changes

• Remove the Yarn package-manager aspect, runtime adapter, configuration hooks, doctor metadata, internal docs, and Yarn-specific tests.
• Preserve Yarn in component usage docs as a supported way to consume published packages, including the yarn add command.
• Use pnpm for Bit-managed installs and keep npm documented only for external package-manager mode or package consumption.
• Remove obsolete package-manager plumbing that no remaining adapter used: the legacy npm/Yarn client config, raw CLI/config argument passthrough, no-op strict/extra-argument settings, and the Yarn-only capsule-cache build flag.
• Keep package managers extensible through registerPackageManager(), the package-manager slot, and arbitrary configured aspect IDs; document and test the custom-ID path.
• Remove Yarn-only dependencies and prune deleted and unreachable records from the pnpm lockfile and generated metadata.

Validation

• oxlint --deny-warnings scopes e2e components
• Structured metadata and pnpm lockfile parsing
• pnpm lockfile reachability check (no orphaned records)
• tsc --noEmit — blocked by the existing clientsClaim type error in scopes/ui-foundation/ui/rspack/rspack.browser.config.ts:126
• Targeted dependency-resolver Mocha spec — the existing Chai 5 ESM/CJS loader failure occurs before assertions in all tests