[Aschenbath]

When corepack install reuses the project's package manager from the cache, it currently prints Adding ... to the cache... again and can omit the stored integrity hash. This change reports <name>@<version>+<hash> is already in the cache for cached installs.

• Return isAlreadyInstalled from both paths of installVersion, and pass that status through the existing ensurePackageManager result. The status is per invocation and is not persisted in .corepack metadata.
• Select the local install message after installation succeeds. Cache hits use the locator with the stored integrity hash; fresh installs retain the existing Adding ... message.
• Cover repeated installs with and without an explicit hash, exact and ranged devEngines specifications, offline cache reuse, cache cleaning, unchanged project metadata, and repeated integrity failures. The integrity failure test checks stdout, matching Clipanion's existing command error handling, and rejects misleading installation or cache-hit messages.

Validation on Node.js 24.20.0:

• Build bundle and regenerate shims: passed.
• tsc --noEmit: passed.
• ESLint across sources, tests, build script, and configuration with --max-warnings=0: passed.
• Related Vitest suites (InstallLocal, corepackUtils, Use, Up, npmRegistryUtils): 35 tests passed, with retries disabled.
• Full Vitest run in replay mode with environment proxy variables removed: 8 files passed, 176 tests passed, 1 expected failure; the live npm key test failed because direct external TLS is unavailable in this environment. Running tests/config.test.ts separately with the external proxy enabled passed (1 test). All 9 files have passed across these runs.

Closes #423

[aduh95]

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