ci: skip cargo-hack feature checks on PRs (keep on merge queue)

[huth-stacks]

What

Skip the cargo-hack feature combination check on PRs. Keep it on merge queue, releases, and manual dispatch.

Why

cargo hack check --all --each-feature takes 13 minutes per PR, checking every feature combination across all 13 workspace crates for Windows and Linux targets. This is valuable for catching feature flag incompatibilities, but:

• Feature-only breakages are rare (most PRs change code, not features)
• The merge queue still runs cargo-hack, catching issues before code reaches develop
• clippy already runs on PRs and catches compilation issues

The Change

In .github/workflows/ci.yml, remove github.event_name == 'pull_request' from cargo-hack-check's condition:

-      github.event_name == 'pull_request' ||

1 line removed from 1 file.

After this change, cargo-hack runs on:

• Merge queue entries
• Release branches
• Manual dispatch
• Pull requests (skipped for speed)

Metrics to Track

Metric	Baseline	Expected
cargo-hack native-targets	13m14s	Skipped (0s)
cargo-hack WASM targets	1m53s	Skipped (0s)
cargo-hack fuzz targets	1m27s	Skipped (0s)

Security Checklist

• No new permissions
• No secrets exposure
• Merge queue still validates feature combinations before merge

Part of CI Optimization Series

PR 10. Expected impact: ~13 minutes off every PR run.