fix: respect conflicting lockfiles when auto-pinning

[Aschenbath]

When COREPACK_ENABLE_AUTO_PIN=1, running another package manager in a project identified by a lockfile could silently pin that other manager in package.json.

Skip automatic pinning when package-lock.json, npm-shrinkwrap.json, pnpm-lock.yaml, or yarn.lock next to the selected manifest belongs to a different manager. The requested command still runs, and corepack use remains available for an explicit selection. Document the behavior and add 13 CLI regressions for all three managers, matching/conflicting lockfiles, multiple lockfiles, nested directories, and explicit selection.

Validation on Node.js 24.20.0:

• Build: passed.
• Full Vitest suite with NOCK_ENV=replay: 9 files passed; 184 tests passed and 1 existing expected failure.
• tsc --noEmit: passed.
• ESLint with --max-warnings=0: passed.

Local test setup isolates inherited proxy settings from CLI fixtures while retaining access for the live npm-key check. Compilation caches were disabled to accommodate the small temporary filesystem. No tests were skipped or modified for these environment adjustments.

Closes #695

[aduh95]

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