Add AGENTS.md

**Is your feature request related to a problem? Please describe.**
AI agents waste tokens rediscovering repo structure on every new conversation.

**Describe the solution you'd like**
Add an AGENTS.md that documents project layout, stack, common commands, and conventions.

**Describe alternatives you've considered**
- READM - too broad, buried under user-facing content
- CONTRIBUTING.md - human-focused, not machine-optimized
- Inline agent configs (`.claude/.opencode/`) - project-agnostic, would need per-agent duplication

**Are you going to work on implementing this?**
- [x] Yes
- [ ] No

**Additional context**
Popularized by agentic coding tools - single file agents parse upfront, skip re-discovery on every turn & consumes less tokens on every run.