ci(release): unblock v0.2.0 publish path

[Lythaeon]

Summary

• switch macOS x86 build runner from macos-13 to macos-15-intel to avoid unsupported runner cancellation
• skip crates publish when the target crate version already exists on crates.io
• keep release finalization flow intact so existing draft v0.2.0 can be finalized with binaries

Why this is needed now

The current main release run failed because:

1. macOS x86 matrix job used an unsupported runner label
2. crates publish attempted slotstrike 0.2.0 even though it already exists

Both conditions prevented the GitHub release job from running.

Validation

• inspected failed run: https://github.com/Lythaeon/slotstrike/actions/runs/22040218842
• confirmed error messages for runner and crates publish
• reviewed updated workflow dependencies/conditions in .github/workflows/release.yml