Add windmill AGENTS guidance for harness workflow

AGENTS.md

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+# Project AGENTS (windmill)
+
+## Startup
+- Load shared global rules from:
+  - `C:/Users/akira/.codex/shared/GLOBAL_RULES.md`
+- Apply project-local rules in this file after shared rules.
+
+## Required Read Order
+1. `HARNESS.md`
+2. `INDEX.md`
+3. `HANDOFF.md`
+4. `SERVER_SETUP.md`
+
+## Working Mode (mandatory)
+- This repository uses harness workflow.
+- Every request must be framed in 3 lines:
+  - Purpose
+  - Target
+  - Done
+- Follow `INDEX.md` work sequence and `HARNESS.md` validation policy.
+
+## Safety Rules (mandatory)
+- Work locally first.
+- Before edits, ensure backup branch exists: `backup/2026-03-05-before-harness`.
+- Implement on working branch (current default: `chore/harness-introduce`).
+- If unexpected unrelated changes are detected, stop and ask.
+
+## Recording Rules
+- Record each task in `state/tasks/<task_id>.md`.
+- Accumulate regression cases in `tests/golden/`.
+