fix(ci): build all companions before release tests

[smackgg]

What changed

• make tag verification reuse the repository's complete bun run check pipeline
• build every declared companion before repository tooling tests
• advance FFmpeg Plugin and Skill to 0.3.1, Xiaoyunque Plugin to 0.3.2, and Registry sequence to 21 because the failed tags are immutable

Root cause

The tag workflow built only the companion selected by the current tag, then ran full repository tests that deterministically package every Plugin. The tests therefore failed whenever the other Plugin companion was absent. Main and pull-request validation did not reproduce this because both use bun run check, which builds all companions first.

Verification

• reproduced all three failed tag runs from their GitHub Actions logs
• CONVAX_FFMPEG_SOURCE_ARCHIVE=<verified ffmpeg-8.1.2.tar.xz> bun run check
• exact tag pack for plugin-ffmpeg-tools-v0.3.1
• exact tag pack for skill-ffmpeg-canvas-v0.3.1
• exact tag pack for plugin-xiaoyunque-generation-v0.3.2
• Registry and Showcase sequence 21 built successfully