fix(ci): restore pull-request benchmark runs

[zrosenbauer]

Summary

Restore pull-request benchmark runs. The CI / Benchmarks workflow was disabled for pull requests because jobs targeting codspeed-macro remained pending indefinitely.

Current state

The workflow remains available through workflow_dispatch, but pull requests no longer exercise it. This avoids permanently pending checks at the cost of losing benchmark feedback.

Acceptance criteria

• A supported runner is selected and documented.
• The pull-request trigger is restored for relevant package, benchmark, and lockfile changes.
• A clean run completes end to end and publishes the expected CodSpeed result.
• Failed or unavailable runners terminate visibly instead of remaining pending indefinitely.