Add AGENTS.md documenting the repo's conventions

Codifies what is currently tribal knowledge or only discoverable by reading CI
config: the view model / view contract, the rule that nothing reads the page,
Clean Code ordering, the three test layers, and reuse-before-you-build.

The root AGENTS.md is the always-loaded minimum and routes to docs/agents/, so
an agent loads only the detail its task needs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -10,6 +10,9 @@ public/config.json
 backend/synapse_tmp/*
 backend/synapse_tmp_othersite/*
 /coverage
+
+# Transient agent scratch files
+/agent-workspace/
 config.json
 
 # Yarn