fix: clean up RC tags for custom tag prefixes on publish

[joshua-temple]

Problem

When an rc is promoted to a full release, publish() calls cleanupRCTags() to delete the superseded RC git tags. parseRCTag hardcoded the v prefix (^(v\d+\.\d+\.\d+)-rc\.(\d+)$), so a repo using a custom tag_prefix (e.g. rel-) produced RC tags like rel-0.1.0-rc.0 that failed the regex. Cleanup skipped them, leaving stale RC tags after publish.

Fix

parseRCTag / isRCTag are now prefix-aware. The base-version capture includes the prefix (^(.*\d+\.\d+\.\d+)-rc\.(\d+)$), so the value compares directly against the published release tag without reconstructing the prefix. The default v, custom prefixes (rel-, release/), and the empty prefix all parse. The $ anchor keeps hotfix-shaped tags (...-rc.N.hotfix.M) inert, and cleanup still only deletes RC tags whose prefixed base matches the published version.

Verification

• TestParseRCTag extended with custom-prefix, empty-prefix, hotfix, and non-numeric cases.
• New TestManager_Publish_CustomTagPrefix: publishing rel-0.1.0 deletes rel-0.1.0-rc.0/1, while rel-0.2.0-rc.0 (different base) and v0.1.0-rc.0 (different prefix) survive.
• go build ./... && go test ./... && golangci-lint run ./... green.

The RC git-tag cleanup path is GitHub-only (gated by isGitHubHost); the Gitea e2e backend lacks the git-data refs API, so a Gitea e2e scenario would not exercise it. Coverage is the targeted unit test plus the real-GitHub validation fleet.