test: fix public mirror baseline assumptions

[icatw]

Summary

• Fixes the SyncEngine conflict fixture so the bare test remote points HEAD at main before cloning.
• Adjusts the public mirror skill-bundle asset guard to require _shared only when source bundles still reference shared placeholders.

Verification

• bun test packages/server/src/sync-engine.test.ts --test-name-pattern 'SyncEngine ConflictStore admission'
  • 3 pass / 0 fail
• bun test packages/server/scripts/build-skill-bundles.test.ts --test-name-pattern 'repo assets'
  • 7 pass / 0 fail

Verification screenshot:

Full-repo check note

This PR fixes two reproducible public-mirror baseline failures. bun run check is still blocked separately by oxlint@1.66.0 panicking with called Result::unwrap() on an Err value / SIGABRT under the repo's current oxlint.config.ts.

[CLAassistant]

All committers have signed the CLA.

[github-actions]

Thanks for the contribution! A maintainer will review and merge your PR. Your commit attribution is preserved as @icatw.

What happens next:

• A maintainer will review your PR.
• If you don't hear back within a few business days, please comment here to nudge — that's the right thing to do, not annoying.
• When your change is accepted, this PR closes automatically. Don't be alarmed when it closes — that's how it merges, and your authorship is preserved.

This comment will be updated as the status changes.