feat(ci): rebuild packages.wheels.dev on manifest merges

[bpamiri]

Summary

• Fires a repository_dispatch event at wheels-dev/wheels on every push to main that touches packages/** or schema/**
• Pairs with the matching trigger in wheels-dev/wheels/.github/workflows/web-deploy.yml (see the companion PR adding the packages.wheels.dev static site)

Manual prep required before first trigger fires

This workflow references a repository secret named NOTIFY_WHEELS_TOKEN. Before merging this PR, create:

1. A fine-grained personal access token:

   • Resource owner: wheels-dev
   • Repository access: Only select repositories -> wheels-dev/wheels
   • Permissions -> Repository -> Contents: Read and write
   • Expiration: 1 year (or whatever rotation cadence works)

2. Add it as a repo secret in this repo (wheels-dev/wheels-packages):

   • Settings -> Secrets and variables -> Actions -> New repository secret
   • Name: NOTIFY_WHEELS_TOKEN
   • Value: the token from step 1

If the secret isn't configured, workflow runs will fail with a visible "missing secret" error - degrade is visible, not silent.

Test plan

• Merge this PR
• Trigger a test manifest change (whitespace-only commit) to confirm the notify workflow runs and the wheels-dev/wheels deploy workflow is triggered
• Check packages.wheels.dev reflects the change within ~3 minutes