fix(cli): report missing npm during install

[cyphercodes]

Description

Fixes #12192.

Adds explicit npm validation before the CLI installer uses npm config get prefix. If Node.js is present but npm is missing or its global prefix cannot be read / resolves to an unsafe value, the installer now prints a targeted Node.js was found but npm is missing or misconfigured error instead of only exiting through the generic cleanup message.

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

Not applicable; this is installer error handling.

Tests

• bash -n extensions/cli/scripts/install.sh
• git diff --check
• Temporary fake-PATH harness for setup_npm_path with no npm available: verified it exits non-zero with the targeted npm diagnostic.
• Temporary installer main-flow harness with fake Node.js v22.22.2 and no npm available: verified it exits non-zero after reporting the targeted npm diagnostic.

[github-actions]

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[cubic-dev-ai]

No issues found across 1 file

[cyphercodes]

I have read the CLA Document and I hereby sign the CLA