fix: edge case in normalizeBeadsRelPath for similar prefixes

Code review caught that ".beads" would incorrectly match prefixes like
".beads-backup". Changed to match ".beads/" (with trailing slash) to
ensure we only match the actual .beads directory.

Added test cases for this edge case.

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

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

internal/syncbranch/worktree_path_test.go

@@ -218,6 +218,16 @@ func TestNormalizeBeadsRelPath(t *testing.T) {
 			input:    ".beads/subdir/file.jsonl",
 			expected: ".beads/subdir/file.jsonl",
 		},
+		{
+			name:     "similar prefix like .beads-backup not matched",
+			input:    "foo/.beads-backup/.beads/issues.jsonl",
+			expected: ".beads/issues.jsonl",
+		},
+		{
+			name:     "only .beads-backup no real .beads unchanged",
+			input:    "foo/.beads-backup/file.txt",
+			expected: "foo/.beads-backup/file.txt",
+		},
 	}
 
 	for _, tt := range tests {