docs(transformer): add AGENTS.md test guidance for agents (#23439)

This PR adds an `AGENTS.md` to `crates/oxc_transformer`. AI agents often try to write tests for transformer regressions inside `crates/oxc_transformer` which is incorrect. These tests should go inside the `tasks/transform_conformance` fixtures instead.

crates/oxc_transformer/AGENTS.md

@@ -0,0 +1,71 @@
+# Coding agent guides for `crates/oxc_transformer`
+
+## Testing transformer behavior
+
+Most transformer behavior regressions should be tested through transform conformance fixtures, not Rust integration tests in this crate.
+
+Use:
+
+```text
+tasks/transform_conformance/tests/<transform-name>/test/fixtures/<case-name>/
+```
+
+Each fixture normally contains:
+
+```text
+input.js
+output.js
+```
+
+Use `input.ts`, `input.tsx`, or `input.jsx` when the regression depends on TypeScript or JSX syntax. The transform options are usually inherited from the nearest `options.json`.
+
+Run a focused fixture with:
+
+```sh
+cargo run -p oxc_transform_conformance -- --filter <transform-name>/test/fixtures/<case-name>
+```
+
+Run all transform conformance tests with:
+
+```sh
+cargo run -p oxc_transform_conformance
+```
+
+If the fixture has an `exec.js` runtime assertion, include `--exec`:
+
+```sh
+cargo run -p oxc_transform_conformance -- --filter <transform-name>/test/fixtures/<case-name> --exec
+```
+
+## OXC-specific regressions
+
+When the case is OXC-specific, put it under that transform's local `oxc` fixture group:
+
+```text
+tasks/transform_conformance/tests/<transform-name>/test/fixtures/oxc/<case-name>/
+```
+
+For example, optional chaining regressions belong under:
+
+```text
+tasks/transform_conformance/tests/babel-plugin-transform-optional-chaining/test/fixtures/oxc/<case-name>/
+```
+
+with:
+
+```text
+input.js
+output.js
+```
+
+or `input.ts` if the case depends on TypeScript syntax. The shared optional chaining options live in:
+
+```text
+tasks/transform_conformance/tests/babel-plugin-transform-optional-chaining/test/fixtures/oxc/options.json
+```
+
+## When to use crate integration tests
+
+Use `crates/oxc_transformer/tests/integrations/` for transformer API behavior, target selection, helper loading, or cross-cutting Rust harness coverage.
+
+Do not add ordinary transform output regressions there when a `tasks/transform_conformance` fixture can cover the behavior.