ci: disable automatic workflow triggers, add workflow_dispatch

[devin-ai-integration]

Summary

Disable automatic triggers in npm.yml (cron + push) and stale.yml (cron) to reduce Actions spend. Job definitions are preserved — workflows can be triggered manually via workflow_dispatch.

Review & Testing Checklist for Human

• Verify npm.yml and stale.yml now only trigger on workflow_dispatch
• Confirm job definitions are unchanged

Notes

This is part of an org-wide effort to reduce GitHub Actions spend by disabling automatic triggers across all repos.

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/7b4b537d6a95428d85ae1cf444f374ce
Requested by: @bsmitches

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[devin-ai-integration]

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