chore: ignore opentree worktree dir and config (#412)

.opentree/ contains git worktrees created by the opentree CLI tool
(https://github.com/axelgar/opentree). These are local developer
artifacts and should not be tracked.

opentree.toml is the per-repo opentree config - also local/personal,
not something contributors need to share.

.gitignore

@@ -152,3 +152,7 @@ temp/
 # Analysis documents (keep local)
 CROSS_DISTRO_ANALYSIS.md
 FIX_PLAN.md
+
+# opentree - local AI worktree orchestration tool (per-developer, not shared)
+.opentree/
+opentree.toml