Add project docs and initial configuration

- PRD.md: full product requirements for the budget app
- CLAUDE.md: repo guidance including td task management workflow
- AGENTS.md: session startup instructions
- .gitignore: comprehensive ignore rules for Node/React/Docker/editor artifacts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,43 @@
+# td / sidecar
+.todos/
+.sidecar/
+.sidecar-agent
+.sidecar-task
+.sidecar-pr
+.sidecar-start.sh
+.sidecar-base
+.td-root
+
+# Dependencies
+node_modules/
+
+# Build output
+dist/
+build/
+.vite/
+
+# Environment
+.env
+.env.local
+.env.*.local
+
+# Logs
+logs/
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Editor
+.vscode/
+.idea/
+*.swp
+*.swo
+
+# Coverage
+coverage/