fix(git-prompt): improve branch name resolution using symbolic-ref

Problem:
In worktrees, enabling reftable caused the gitfast prompt to sometimes show “.invalid” for the branch name, e.g. `git:(.invalid|SPARSE)`. The cause was gitfast’s __git_ps1 reading $GIT_DIR/HEAD (and sequencer head-name files) directly and attempting to derive the ref name by string slicing. That approach is brittle with worktrees and newer ref backends like reftable.

What changed:
- Prefer `git symbolic-ref --short HEAD` to resolve the branch name.
- Also use `--short` when HEAD is a symlink.
- Keep existing fallbacks (describe/detached SHA) unchanged.

Why this is correct:
- Delegates ref resolution to Git’s ref API, which understands:
  - multiple worktrees (`.git/worktrees/<name>/HEAD`)
  - all ref storage backends (loose, packed, reftable)
  - edge cases around in-progress operations and symlinks
- Avoids parsing internal files that can legitimately be non-canonical or transient (e.g., head-name during rebase, or placeholders that appear as “.invalid” with reftable+worktrees).

Impact:
- Correct branch names in worktrees regardless of ref backend (fixes “.invalid”).
- Detached HEAD behavior remains: still falls back to describe/short SHA.
- `|SPARSE` logic unchanged and still driven by `core.sparseCheckout`.
- Backwards-compatible: older Git that supports `symbolic-ref --short` works; fallbacks still apply if not a branch.

Rationale:
Ref backends like reftable decouple refs from the filesystem layout that gitfast was implicitly depending on. Using Git’s plumbing for branch resolution is the robust, future-proof approach.

plugins/gitfast/git-prompt.sh

@@ -474,9 +474,12 @@ __git_ps1 ()
 
 		if [ -n "$b" ]; then
 			:
+		# Prefer symbolic-ref short name resolution (works well with worktrees)
+		elif symref_short="$(git symbolic-ref --quiet --short HEAD 2>/dev/null)" && [ -n "$symref_short" ]; then
+			b="$symref_short"
 		elif [ -h "$g/HEAD" ]; then
 			# symlink symbolic ref
-			b="$(git symbolic-ref HEAD 2>/dev/null)"
+			b="$(git symbolic-ref --short HEAD 2>/dev/null)"
 		else
 			local head=""
 			if ! __git_eread "$g/HEAD" head; then