docs: add agent and domain guidance

docs/agents/domain.md

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+# Domain docs
+
+Engineering skills must consult this repository's domain documentation before
+exploring code.
+
+## Read before exploring
+
+- Read `CONTEXT.md` at the repository root.
+- If `CONTEXT-MAP.md` exists, use it to find the `CONTEXT.md` files relevant to
+  the task.
+- Read ADRs under `docs/adr/` that affect the area you will change. In a
+  multi-context repository, also check `src/<context>/docs/adr/`.
+
+If these files do not exist, proceed without comment. Do not suggest creating
+them before the project resolves domain terms or architectural decisions. The
+`/domain-modeling` skill, reached through `/grill-with-docs` and
+`/improve-codebase-architecture`, creates them when needed.
+
+## File structure
+
+This repository uses the single-context layout:
+
+```text
+/
+├── CONTEXT.md
+├── docs/adr/
+│   ├── 0001-event-sourced-orders.md
+│   └── 0002-postgres-for-write-model.md
+└── src/
+```
+
+A repository with multiple bounded contexts can use this layout:
+
+```text
+/
+├── CONTEXT-MAP.md
+├── docs/adr/
+└── src/
+    ├── ordering/
+    │   ├── CONTEXT.md
+    │   └── docs/adr/
+    └── billing/
+        ├── CONTEXT.md
+        └── docs/adr/
+```
+
+## Use glossary terms
+
+Use the terms defined in `CONTEXT.md` in issue titles, refactor proposals,
+hypotheses, and test names. Do not substitute synonyms that the glossary
+rejects.
+
+If a needed concept is absent, reconsider whether the project uses that concept.
+If it represents a real gap, record it for `/domain-modeling`.
+
+## Flag ADR conflicts
+
+Call out any proposal that conflicts with an ADR:
+
+> _Contradicts ADR-0007 (event-sourced orders), but worth reopening because…_