ci: consolidate test and coverage jobs

[soulteary]

Summary

• upload Codecov results from the existing race-enabled Quality test instead of running a second full test job
• remove the redundant coverage workflow
• stop running the Release workflow on every main push while preserving tag and manual triggers
• rely on setup-go's module cache instead of a second cache action in Release

Verification

• parsed all remaining workflow files as YAML
• go test ./...
• git diff --check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Upload coverage before unrelated checks

When go vet, the govulncheck installation, or the vulnerability scan fails after tests successfully produce coverage.out, GitHub's implicit success() condition skips this upload. The deleted Codecov workflow uploaded whenever its tests succeeded, so this consolidation now leaves Codecov stale during unrelated check failures; move the upload directly after the test or otherwise condition it on the coverage-producing step.

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