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>

AGENTS.md

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+## MANDATORY: Use td for Task Management
+
+Run td usage --new-session at conversation start (or after /clear). This tells you what to work on next.
+
+Sessions are automatic (based on terminal/agent context). Optional:
+- td session "name" to label the current session
+- td session --new to force a new session in the same context
+
+Use td usage -q after first read.