chore: move pnpm settings to pnpm-workspace.yaml

[viceice]

Changes

Moves the pnpm settings from .npmrc to pnpm-workspace.yaml and removes .npmrc.

savePrefix: '' is dropped: with saveExact: true, pnpm already saves exact versions (checked with pnpm add on pnpm 12.12.0).

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Claude Code (Claude Opus 5.5) made the change and checked it.

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