Agents (#4324)

## Motivation and Context
Just dropping in something basic. we can iterate on this until AI is
decent at working in this codebase.

----

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choice._

Author: rcoh

AGENTS.md

@@ -0,0 +1,130 @@
+# Smithy-rs AI Agent Guide
+
+## Package Layout
+
+- **`codegen-core/`** - Shared codegen
+- **`codegen-server/`** - Server codegen
+- **`codegen-client/`** - Client codegen
+- **`rust-runtime/`** - Runtime libraries
+- **`codegen-server-test/`** - Server integration tests
+
+Protocol files: `codegen-{core,server}/.../protocols/`
+
+## preludeScope: Rust Prelude Types
+
+**Always use `preludeScope` for Rust prelude types:**
+
+```kotlin
+rustTemplate(
+    "let result: #{Result}<#{String}, #{Error}> = #{Ok}(value);",
+    *preludeScope,  // Provides Result, String, Ok
+    "Error" to myErrorType
+)
+```
+
+❌ Wrong: `"let result: Result<String, Error> = Ok(value);"`
+✅ Correct: Use `*preludeScope` in templates
+
+## RuntimeType and Dependencies
+
+`RuntimeType` objects contain:
+- **`path`**: Rust path (e.g., `"::mime::Mime"`)
+- **`dependency`**: `CargoDependency` or `InlineDependency`
+
+Using a `RuntimeType` automatically adds its dependency to `Cargo.toml`.
+
+### Creating RuntimeTypes
+
+```kotlin
+// Pre-defined dependencies
+val Mime = CargoDependency.Mime.toType()
+val Bytes = CargoDependency.Bytes.toType().resolve("Bytes")
+
+// Runtime crates
+val smithyTypes = RuntimeType.smithyTypes(runtimeConfig)
+```
+
+### Always Use Symbols
+
+❌ Wrong: `rust("const MIME: ::mime::Mime = ::mime::APPLICATION_JSON;")`
+✅ Correct: `rustTemplate("const MIME: #{Mime}::Mime = #{Mime}::APPLICATION_JSON;", "Mime" to RuntimeType.Mime)`
+
+## RuntimeType.forInlineFun: Lazy Generation
+
+Code is only generated if used. `forInlineFun` enables lazy generation:
+
+```kotlin
+val mimeType = RuntimeType.forInlineFun("APPLICATION_JSON", module) {
+    rustTemplate(
+        "pub const APPLICATION_JSON: #{Mime}::Mime = #{Mime}::APPLICATION_JSON;",
+        "Mime" to RuntimeType.Mime
+    )
+}
+```
+
+⚠️ **Footgun**: Name collisions mean only one implementation gets generated.
+
+## Testing
+
+### Integration Tests
+Test actual generated code, not just codegen logic:
+
+```kotlin
+serverIntegrationTest(model) { codegenContext, rustCrate ->
+    rustCrate.testModule {
+        tokioTest("test_accept_header") {
+            rustTemplate("""
+                let request = ::http::Request::builder()
+                    .header("Accept", "application/cbor")
+                    .body(Body::empty()).unwrap();
+                let result = MyInput::from_request(request).await;
+                result.expect("should accept valid header");
+            """)
+        }
+    }
+}
+```
+
+### Running Tests
+
+**Codegen tests:**
+```bash
+./gradlew test --tests "*MyTest*"
+./gradlew codegen-server-test:assemble --quiet
+```
+
+**Debug failing tests:**
+```bash
+# Remove --quiet to see failure details
+./gradlew :codegen-core:test --tests "*InlineDependencyTest*"
+# Extract just the error from HTML report (avoid HTML markup pollution)
+grep -A 5 "AssertionError\|Exception" codegen-core/build/reports/tests/test/classes/software.amazon.smithy.rust.codegen.core.rustlang.InlineDependencyTest.html
+```
+
+**Runtime tests:**
+```bash
+cd rust-runtime && cargo test --quiet -p aws-smithy-types
+```
+
+**Protocol tests:**
+```bash
+./gradlew codegen-client-test:assemble --quiet
+cd codegen-client-test/build/smithyprojections/codegen-client-test/rest_xml_extras/rust-client-codegen
+cargo test --quiet
+```
+
+## Viewing Generated Code
+
+Generated code appears in:
+```
+codegen-server-test/build/smithyprojections/codegen-server-test/SERVICE_NAME/rust-server-codegen/
+```
+
+Enable debug comments:
+```kotlin
+serverIntegrationTest(model, IntegrationTestParams(
+    additionalSettings = ServerAdditionalSettings.builder()
+        .generateCodegenComments()  // Adds Kotlin source line comments
+        .toObjectNode()
+)) { /* test code */ }
+```