Add AGENTS.md with repo conventions

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S9abShYK5RMmbeuZurGH3b

Author: claude

AGENTS.md

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+# AGENTS.md
+
+Coder Docker images (fx, php, nodejs, golang, python).
+
+## Writing style
+
+- Write in ASD-STE100 (Simplified Technical English).
+- Follow Zinsser's four principles of quality writing:
+  1. Simplicity
+  2. Brevity
+  3. Clarity
+  4. Humanity
+
+## Conventions
+
+- Pin every tool version with an `ENV *_VERSION` variable in each Dockerfile.
+- Keep Dockerfile comments very short and simple.
+- No unparenthesized `|| true` in RUN chains — it can silently skip the rest of the build.
+- Smoke tests live in the Makefile (`make test DOCKER_VARIANT=<variant>`).

CLAUDE.md

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+# CLAUDE.md
+
+See [AGENTS.md](AGENTS.md).