chore: migrate .github/workflows/publish.yaml from PAT to GitHub App auth

[manjeet-fetchai]

Summary

Migrates .github/workflows/publish.yaml from PAT auth to GitHub App auth (Wave 1 of Phase 1 plan).

What changed

• Secret replaced: GH_PAT (Bearer token in repository_dispatch curl)
• Replacement: actions/create-github-app-token@v1 using fetchai-dispatch App (FETCHAI_DISPATCH_APP_ID / FETCHAI_DISPATCH_APP_KEY)
• New step added: Generate GitHub App token inserted before the dispatch step
• Token reference updated: curl Authorization header now uses ${{ steps.app-token.outputs.token }}

What was NOT changed

• No reformatting, reordering, or other edits
• GH_PAT secret remains active until all consumers are migrated and verified

Verification

After merge, push a dummy commit to the default branch and confirm:

1. The Generate GitHub App token step succeeds
2. The Repository Dispatch curl returns HTTP 204
3. The target infra repo Actions tab shows a triggered dispatch run

Ref: docs/plans/phase1-actions-workflows.md