feat: add monorepo sync notification workflow

[mattapperson]

🔄 Automated Monorepo Sync

This PR adds a workflow that automatically notifies the openrouter-web monorepo when changes are pushed to main in this repository.

How It Works

typescript-sdk (this repo)          openrouter-web (monorepo)
├─ Commit pushed to main            ├─ Receives repository_dispatch
├─ notify-monorepo.yaml triggers    ├─ Syncs typescript subtree
├─ Sends webhook event ────────────>├─ Creates PR with changes
                                     └─ Auto-merges if CI passes

What's Added

File: .github/workflows/notify-monorepo.yaml

Trigger: Runs automatically on every push to main

Action: Sends a repository_dispatch event to the monorepo to trigger subtree sync

Setup Required

After merging this PR, an admin needs to add a secret:

1. Go to Settings → Secrets and variables → Actions
2. Click New repository secret
3. Name: MONOREPO_SYNC_TOKEN
4. Value: GitHub Personal Access Token with repo scope
5. Click Add secret

The token allows this repo to trigger workflows in the monorepo.

Benefits

• ✅ Automatic sync of changes to monorepo
• ✅ Changes reviewed via PR before merging to monorepo
• ✅ No manual intervention needed
• ✅ Fast propagation of fixes and features

Testing

After setup is complete:

1. Push a commit to main in this repo
2. Check Actions tab - "Notify Monorepo of Changes" should run
3. Check openrouter-web Actions - "Sync Subtrees" should trigger
4. A PR should be created in openrouter-web with the changes

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Related: OpenRouterTeam/openrouter-web#9617