chore(agents): add AGENTS.md/CLAUDE.md rules for AI coding agents

Any AI agent working in this repo (Claude Code, Codex, Kimi, or any
other) must stay schema-driven and never hardcode object types, field
names, filters, or columns as a shortcut. AGENTS.md is the index (tech
stack, commands, universal rules); CLAUDE.md just points Claude Code at
it so the same rules apply without duplicating content. The detailed
schema-fidelity rule lives in .agents/rules/.

.gitignore previously excluded *.md except README/CHANGELOG, which
would have hidden these files (and SCHEMA_DEVIATIONS.md) from git.

.gitignore

@@ -27,6 +27,10 @@ scripts/
 *.md
 !README.md
 !CHANGELOG.md
+!AGENTS.md
+!CLAUDE.md
+!SCHEMA_DEVIATIONS.md
+!.agents/rules/*.md
 /SPEC-*
 # Tool-generated artifacts during agent sessions
 .playwright-mcp/