fix(ci): run CI only for pull requests

[miguelg719]

Summary

• stop running the full CI workflow again after every merge to main
• keep pull request CI unchanged
• preserve workflow_dispatch for trusted external-contributor handoff runs

This removes the main push trigger introduced in #2671. Shared protocol changes can activate every SDK filter, so merges such as #2790 repeated the complete test matrix after it had already passed on the PR.

Test plan

• actionlint -ignore SC2046 .github/workflows/ci.yml
• git diff --check

---

Summary by cubic

Stop running the CI workflow on pushes to main to avoid duplicate runs after merges; keep CI behavior for pull requests and manual dispatches unchanged.

• Removes the push trigger on main; retains pull_request and workflow_dispatch (with pull_request_number input) for trusted handoffs.
• Pushes to main no longer run CI. If any branch protection requires push-based checks, update it to rely on PR-based statuses.

^{Written for commit 3d4a378. Summary will update on new commits.}

[changeset-bot]

⚠️ No Changeset found

Latest commit: 3d4a378

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[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}