fix(merge): fix tombstone handling edge cases (bd-ki14, bd-ig5, bd-6x5, bd-1sn)

- bd-ki14: Preserve tombstones when other side implicitly deleted
  In merge3WayWithTTL(), implicit deletion cases now check if the
  remaining side is a tombstone and preserve it instead of dropping.

- bd-ig5: Remove duplicate constants from merge package
  StatusTombstone, DefaultTombstoneTTL, and ClockSkewGrace now
  reference the types package to avoid duplication.

- bd-6x5: Handle empty DeletedAt in mergeTombstones()
  Added explicit handling for edge cases where one or both tombstones
  have empty DeletedAt fields with deterministic behavior.

- bd-1sn: Copy tombstone fields in mergeIssue() safety fallback
  When status becomes tombstone via mergeStatus safety fallback,
  tombstone fields are now copied from the appropriate side.

Added comprehensive tests for all fixed edge cases.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/merge/merge.go

@@ -33,6 +33,8 @@ import (
 	"fmt"
 	"os"
 	"time"
+
+	"github.com/steveyegge/beads/internal/types"
 )
 
 // Issue represents a beads issue with all possible fields
@@ -238,14 +240,14 @@ func makeKey(issue Issue) IssueKey {
 	}
 }
 
-// StatusTombstone is the status value for soft-deleted issues
-const StatusTombstone = "tombstone"
+// bd-ig5: Use constants from types package to avoid duplication
+const StatusTombstone = string(types.StatusTombstone)
 
-// DefaultTombstoneTTL is the default time-to-live for tombstones (30 days)
-const DefaultTombstoneTTL = 30 * 24 * time.Hour
-
-// ClockSkewGrace is added to TTL to handle clock drift between machines
-const ClockSkewGrace = 1 * time.Hour
+// Alias TTL constants from types package for local use
+var (
+	DefaultTombstoneTTL = types.DefaultTombstoneTTL
+	ClockSkewGrace      = types.ClockSkewGrace
+)
 
 // IsTombstone returns true if the issue has been soft-deleted
 func IsTombstone(issue Issue) bool {
@@ -425,11 +427,21 @@ func merge3WayWithTTL(base, left, right []Issue, ttl time.Duration) ([]Issue, []
 			result = append(result, merged)
 		} else if inBase && inLeft && !inRight {
 			// Deleted in right (implicitly), maybe modified in left
+			// bd-ki14: Check if left is a tombstone - tombstones must be preserved
+			if leftTombstone {
+				result = append(result, leftIssue)
+				continue
+			}
 			// RULE 2: deletion always wins over modification
 			// This is because deletion is an explicit action that should be preserved
 			continue
 		} else if inBase && !inLeft && inRight {
 			// Deleted in left (implicitly), maybe modified in right
+			// bd-ki14: Check if right is a tombstone - tombstones must be preserved
+			if rightTombstone {
+				result = append(result, rightIssue)
+				continue
+			}
 			// RULE 2: deletion always wins over modification
 			// This is because deletion is an explicit action that should be preserved
 			continue
@@ -447,8 +459,29 @@ func merge3WayWithTTL(base, left, right []Issue, ttl time.Duration) ([]Issue, []
 
 // mergeTombstones merges two tombstones for the same issue.
 // The tombstone with the later deleted_at timestamp wins.
+//
+// bd-6x5: Edge cases for empty DeletedAt:
+//   - If both empty: left wins (arbitrary but deterministic)
+//   - If left empty, right not: right wins (has timestamp)
+//   - If right empty, left not: left wins (has timestamp)
+//
+// Empty DeletedAt shouldn't happen in valid data (validation catches it),
+// but we handle it defensively here.
 func mergeTombstones(left, right Issue) Issue {
-	// Use later deleted_at as the authoritative tombstone
+	// bd-6x5: Handle empty DeletedAt explicitly for clarity
+	if left.DeletedAt == "" && right.DeletedAt == "" {
+		// Both invalid - left wins as tie-breaker
+		return left
+	}
+	if left.DeletedAt == "" {
+		// Left invalid, right valid - right wins
+		return right
+	}
+	if right.DeletedAt == "" {
+		// Right invalid, left valid - left wins
+		return left
+	}
+	// Both valid - use later deleted_at as the authoritative tombstone
 	if isTimeAfter(left.DeletedAt, right.DeletedAt) {
 		return left
 	}
@@ -494,6 +527,30 @@ func mergeIssue(base, left, right Issue) (Issue, string) {
 	// Merge dependencies - combine and deduplicate
 	result.Dependencies = mergeDependencies(left.Dependencies, right.Dependencies)
 
+	// bd-1sn: If status became tombstone via mergeStatus safety fallback,
+	// copy tombstone fields from whichever side has them
+	if result.Status == StatusTombstone {
+		// Prefer the side with more recent deleted_at, or left if tied
+		if isTimeAfter(left.DeletedAt, right.DeletedAt) {
+			result.DeletedAt = left.DeletedAt
+			result.DeletedBy = left.DeletedBy
+			result.DeleteReason = left.DeleteReason
+			result.OriginalType = left.OriginalType
+		} else if right.DeletedAt != "" {
+			result.DeletedAt = right.DeletedAt
+			result.DeletedBy = right.DeletedBy
+			result.DeleteReason = right.DeleteReason
+			result.OriginalType = right.OriginalType
+		} else if left.DeletedAt != "" {
+			result.DeletedAt = left.DeletedAt
+			result.DeletedBy = left.DeletedBy
+			result.DeleteReason = left.DeleteReason
+			result.OriginalType = left.OriginalType
+		}
+		// Note: if neither has DeletedAt, tombstone fields remain empty
+		// This represents invalid data that validation should catch
+	}
+
 	// All field conflicts are now auto-resolved deterministically
 	return result, ""
 }