[codex] fix upgrade prompt for git describe versions

[youngbeom-shin]

Summary

• Treat local git describe versions such as v0.2.9-4-gd1e3c28-dirty as their base release when checking for updates.
• Keep real prerelease versions such as v0.2.9-rc.1 eligible to update to the final release.
• Add regression coverage for both paths.

Root Cause

The upgrade checker parsed git describe output as a semver prerelease. That made a local build based on v0.2.9 compare lower than the official v0.2.9 release, so the Web UI showed Update & Restart even when the release was already current.

Validation

• env GOCACHE=/Users/shenyongfan/OpenCSG/csgclaw/.gocache go test ./internal/upgrade
• env GOCACHE=/Users/shenyongfan/OpenCSG/csgclaw/.gocache go test ./cli/upgrade
• env GOCACHE=/Users/shenyongfan/OpenCSG/csgclaw/.gocache go test ./internal/api -run 'TestHandleUpgrade|TestIMEventsIncludesUpgradeStatus'