fix(ci): harden release fetch check and automate releases on main

[indykish]

Summary

• fix verify-fetchable by using a deterministic temp workspace and valid Zig project files
• upgrade softprops/action-gh-release from v1 to v2
• switch release workflow trigger to merges on main
• derive release tag from VERSION and use that tag consistently for release + fetch verification

Verification

• actionlint passed for .github/workflows/release.yml

Notes

• release now runs automatically on main merges and publishes v<VERSION>

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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