Fix substring bug in dependency tree cycle detection (#159)

* Add .worktrees/ to .gitignore

Prevents git worktree contents from being tracked in the repository.

* Fix substring bug in dependency tree cycle detection

The cycle detection in GetDependencyTree() was using a simple substring
match which incorrectly flagged valid nodes as cycles. For example,
"bd-1" would be blocked because "bd-10" contains "bd-1" as a substring.

This bug affects any beads project where issue IDs contain each other as
substrings (BD-1/BD-10, ISSUE-1/ISSUE-10, etc).

Changed from:
  AND t.path NOT LIKE '%' || i.id || '%'

To delimiter-aware checks that respect the → separator:
  AND t.path != i.id
  AND t.path NOT LIKE i.id || '→%'
  AND t.path NOT LIKE '%→' || i.id || '→%'
  AND t.path NOT LIKE '%→' || i.id

This ensures we only match complete issue IDs, not substrings.

Added TestGetDependencyTree_SubstringBug to demonstrate and prevent
regression of this issue. The test creates a chain from bd-10 to bd-1
and verifies all nodes appear in the dependency tree.

Discovered while testing dependency tree visualization with bd-1/bd-10.

.gitignore

@@ -53,3 +53,6 @@ result
 
 # GoReleaser build artifacts
 dist/
+
+# Git worktrees
+.worktrees/