feat: add --skip-install to use and up

[Aschenbath]

Add --skip-install to corepack use and corepack up so projects can update packageManager without installing project dependencies. Package-manager resolution, download verification, integrity hashes, and devEngines validation still run.

Document the option and add CLI regressions covering both commands, default and explicit installation behavior, offline reuse of the downloaded manager, signature failures, and manifest validation.

Validation on Node.js 24.20.0:

• Build (esbuild and mkshims.ts): passed.
• Full Vitest suite with NOCK_ENV=replay: 9 files passed; 180 tests passed and 1 existing expected failure.
• tsc --noEmit: passed.
• ESLint: passed.

The local test runner isolates inherited proxy settings from CLI fixtures while retaining proxy access for the live npm-key check. Compile caches were redirected out of this environment's small temporary filesystem. No tests were skipped or modified for these environment adjustments.

Closes #737

[aduh95]

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