fix: respect hierarchy.max-depth config setting (GH#995) (#997)

* fix: respect hierarchy.max-depth config setting (GH#995)

The hierarchy.max-depth config setting was being ignored because storage
implementations had the depth limit hardcoded to 3. This fix:

- Registers hierarchy.max-depth default (3) in config initialization
- Adds hierarchy.max-depth to yaml-only keys for config.yaml storage
- Updates SQLite and Memory storage to read max depth from config
- Adds validation to reject hierarchy.max-depth values < 1
- Adds tests for configurable hierarchy depth

Users can now set deeper hierarchies:
  bd config set hierarchy.max-depth 10

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* refactor: extract shared CheckHierarchyDepth function (GH#995)

- Extract duplicated depth-checking logic to types.CheckHierarchyDepth()
- Update sqlite and memory storage backends to use shared function
- Add t.Cleanup() for proper test isolation in sqlite test
- Add equivalent test coverage for memory storage backend
- Add comprehensive unit tests for CheckHierarchyDepth function

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/hash_ids.go

@@ -4,7 +4,9 @@ import (
 	"context"
 	"database/sql"
 	"fmt"
-	"strings"
+
+	"github.com/steveyegge/beads/internal/config"
+	"github.com/steveyegge/beads/internal/types"
 )
 
 // getNextChildNumber atomically increments and returns the next child counter for a parent issue.
@@ -48,19 +50,18 @@ func (s *SQLiteStorage) GetNextChildID(ctx context.Context, parentID string) (st
 			return "", fmt.Errorf("parent issue %s does not exist and could not be resurrected from JSONL history", parentID)
 		}
 	}
-	
-	// Calculate current depth by counting dots
-	depth := strings.Count(parentID, ".")
-	if depth >= 3 {
-		return "", fmt.Errorf("maximum hierarchy depth (3) exceeded for parent %s", parentID)
+
+	// Check hierarchy depth limit (GH#995)
+	if err := types.CheckHierarchyDepth(parentID, config.GetInt("hierarchy.max-depth")); err != nil {
+		return "", err
 	}
-	
+
 	// Get next child number atomically
 	nextNum, err := s.getNextChildNumber(ctx, parentID)
 	if err != nil {
 		return "", err
 	}
-	
+
 	// Format as parentID.counter
 	childID := fmt.Sprintf("%s.%d", parentID, nextNum)
 	return childID, nil