Phase 1: Add topological sorting to fix import ordering

- Add sort.go with depth-based utilities (GetHierarchyDepth, SortByDepth, GroupByDepth)
- Sort issues by hierarchy depth before batch creation
- Create in depth-order batches (0→1→2→3)
- Fixes latent bug: parent-child pairs in same batch could fail if wrong order
- Comprehensive tests for all sorting functions
- Closes bd-37dd, bd-3433, bd-8b65

Part of bd-d19a (Fix import failure on missing parent issues)

Amp-Thread-ID: https://ampcode.com/threads/T-44a36985-b59c-426f-834c-60a0faa0f9fb
Co-authored-by: Amp <amp@ampcode.com>

internal/importer/sort.go

@@ -0,0 +1,44 @@
+package importer
+
+import (
+	"sort"
+	"strings"
+
+	"github.com/steveyegge/beads/internal/types"
+)
+
+// GetHierarchyDepth returns the depth of a hierarchical issue ID.
+// Depth is determined by the number of dots in the ID.
+// Examples:
+//   - "bd-abc123" → 0 (top-level)
+//   - "bd-abc123.1" → 1 (one level deep)
+//   - "bd-abc123.1.2" → 2 (two levels deep)
+func GetHierarchyDepth(id string) int {
+	return strings.Count(id, ".")
+}
+
+// SortByDepth sorts issues by hierarchy depth (shallow to deep) with stable sorting.
+// Issues at the same depth are sorted by ID for deterministic ordering.
+// This ensures parent issues are processed before their children.
+func SortByDepth(issues []*types.Issue) {
+	sort.SliceStable(issues, func(i, j int) bool {
+		depthI := GetHierarchyDepth(issues[i].ID)
+		depthJ := GetHierarchyDepth(issues[j].ID)
+		if depthI != depthJ {
+			return depthI < depthJ
+		}
+		return issues[i].ID < issues[j].ID
+	})
+}
+
+// GroupByDepth groups issues into buckets by hierarchy depth.
+// Returns a map where keys are depth levels and values are slices of issues at that depth.
+// Maximum supported depth is 3 (as per beads spec).
+func GroupByDepth(issues []*types.Issue) map[int][]*types.Issue {
+	groups := make(map[int][]*types.Issue)
+	for _, issue := range issues {
+		depth := GetHierarchyDepth(issue.ID)
+		groups[depth] = append(groups[depth], issue)
+	}
+	return groups
+}