fix(install): reload user config after preinstall

[arhxam]

What / Why

npm 12 runs a local root preinstall before Arborist fetches dependencies, but npm install currently freezes its Arborist options before that hook. A credential helper can therefore replace an expired token in the configured user .npmrc, yet the active install still sends the old token and fails; only a second invocation succeeds.

Change

• Add Config#reload(where) for already-loaded file-backed config layers. Reloading clears removed values, preserves the existing precedence chain, and invalidates flattened options.
• Reload the user config after a successful local root preinstall and derive Arborist/script-policy options from the refreshed config.
• Keep installs with arguments, global installs, and --ignore-scripts behavior unchanged.
• Document the reload API and cover replacement, removal, environment precedence, flattened options, command integration, and the new debug-log event.

Testing

• node ../../bin/npm-cli.js test -- --no-coverage from workspaces/config
• node . run test -- test/lib/commands/install.js --no-coverage
• node . run test
• Local authenticated-registry reproduction with an expired user token: root preinstall replaced the token, then both the packument and tarball requests used the fresh bearer token and the first npm install completed.

References

Fixes #9853

AI assistance

This change was developed with AI assistance. I reproduced the stale-token behavior against the current latest branch, reviewed the credential/config precedence path, and ran the checks above locally.