chore: add gitignore and finalize initial plan

- .gitignore for .NET output, IDE files, runtime artifacts (todo.db, worktrees, sandbox, logs)
- docs/plan.md: .NET worker spawning Claude CLI, per-list working_dir + git worktrees, 3NF schema, SignalR IPC, Windows-Service deployment path

.gitignore

@@ -0,0 +1,63 @@
+# .NET build output
+bin/
+obj/
+[Bb]uild/
+[Bb]uildResults/
+[Oo]ut/
+[Ll]og/
+[Ll]ogs/
+
+# Visual Studio / Rider / VS Code
+.vs/
+.vscode/
+.idea/
+*.suo
+*.user
+*.userosscache
+*.sln.docstates
+*.userprefs
+
+# Test results
+[Tt]est[Rr]esult*/
+*.coverage
+*.coveragexml
+coverage*.json
+coverage*.xml
+coverage*.info
+
+# Publish output
+publish/
+*.[Pp]ublish.xml
+PublishProfiles/
+
+# NuGet
+*.nupkg
+*.snupkg
+.nuget/
+packages/
+!packages/build/
+project.lock.json
+project.fragment.lock.json
+artifacts/
+
+# Avalonia / XAML designer
+*.designer.cs
+
+# Project-specific
+*.db
+*.db-shm
+*.db-wal
+worker.config.json
+sandbox/
+.claudedo-worktrees/
+
+# OS
+Thumbs.db
+ehthumbs.db
+Desktop.ini
+.DS_Store
+
+# Misc
+*.log
+*.tmp
+*.bak