Isolate pnpm readiness installs

[prineycom]

Summary

• isolate pnpm bootstrap from an enclosing workspace for standalone worktrees
• preserve native workspace installs when a worktree has its own pnpm-workspace.yaml
• cover both recipes in readiness tests

Verification

• node --experimental-strip-types --no-warnings --test test/ready.test.ts
• pnpm exec tsc --noEmit
• pnpm ready YM-214 succeeds in the nested task worktree
• full suite: 314/316; two existing ship-confirmation assertions reflect the local shipConfirmation=false runtime setting