docs: scope AGENTS instructions by directory (#2083)

crates/config/AGENTS.md

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+# Config Crate Instructions
+
+Applies to `crates/config/`.
+
+## Environment Variable Naming
+
+- Global configuration variables must use flat `RUSTFS_*` names.
+- Do not introduce module-segmented names such as `RUSTFS_CONFIG_*`.
+
+Canonical examples:
+
+- `RUSTFS_REGION`
+- `RUSTFS_ADDRESS`
+- `RUSTFS_VOLUMES`
+- `RUSTFS_LICENSE`
+- `RUSTFS_SCANNER_ENABLED`
+- `RUSTFS_HEAL_ENABLED`
+
+## Compatibility
+
+- Deprecated aliases must keep warning behavior.
+- Document aliases in `crates/config/README.md`.
+- Any alias change should include tests for both canonical and deprecated forms.
+
+## Source of Truth
+
+- Constants: `crates/config/src/constants/app.rs`
+- Naming conventions: `crates/config/README.md#environment-variable-naming-conventions`