chore: standardize agent docs and CLAUDE setup (#4)

## Summary
- standardize to root-level AGENTS.md
- set CLAUDE.md symlink -> AGENTS.md
- add docs/ARCHITECTURE.md
- add docs/agents/PLAN.md, ASSUMPTIONS.md, notes/.gitkeep
- remove stale .claude setup where present

## Scope
Only docs/agent-setup cleanup.

Author: glitch418x

.claude/CLAUDE.md AGENTS.md.claude/CLAUDE.md renamed to AGENTS.md

@@ -0,0 +1 @@
+AGENTS.md