gitproto: drop peeled "^{}" refs from AdvRefsToSlice

Pushes against any v1 target were failing with HTTP 400 "invalid reference name: refs/tags/<X>^{}" whenever the planner scheduled a delete on a repo containing annotated tags. Affects every mode whose planner can produce deletes (replicate always, sync with --prune); bootstrap and no-prune sync are latently affected via extra have lines but don't fail at the wire level.

v1 target advertises annotated tags as TWO lines: refs/tags/X <tag-hash> refs/tags/X^{} <commit-hash> ← wire metadata, not a ref

go-git v6 alpha.3 packp.AdvRefs stores peeled lines inline in References with the "^{}" suffix preserved — there is no separate Peeled map. ResolvedReferences() copies them verbatim, so RefHashMap put refs/tags/X^{} into target.refMap, and the planner scheduled a delete for it:

The v2 ls-refs path returns peel info as an attribute and decodeV2LSRefs already discards it, so the bug only ever surfaced on v1.

Fix at the v1-to-slice boundary: AdvRefsToSlice now drops any entry where Name().IsPeeled(). It's the single chokepoint — nothing else reads adv.References directly — and the invariant "this function never returns a name ending in ^{}" is trivial to state, test, and review.