docs(formatter_core): clarify idempotency test infra (#26069)

AGENTS.md

@@ -186,11 +186,7 @@ fn test() {
 
 #### oxc_formatter (and other `oxc_formatter_*` language crates)
 
-- **Fixture tests** (`tests/fixtures/`) and **Prettier conformance** (`tests/conformance.rs`), both part of `cargo test`
-- **Command**: `cargo test -p <crate>` (conformance only: `--test conformance`)
-- **Debug**: `PRETTIER_FILTER=<name> cargo test -p <crate> --test conformance -- --nocapture`
-- Conformance compares output with Prettier's snapshots (suite self-provisioned by `oxc_formatter_tests`); reports are pinned with `insta`
-- Both harnesses re-format every output and record idempotency violations in the snapshots/reports (tracked, not asserted); adding a fixture automatically covers idempotency
+- Read the target crate's `AGENTS.md`; it includes the shared formatter policy plus language-specific rules and verification commands
 
 #### oxc_minifier
 
@@ -273,7 +269,6 @@ These test suites are pre-cloned and ready to search:
 - **Test262** (`tasks/coverage/test262/`) - ECMAScript spec compliance
 - **Babel** (`tasks/coverage/babel/`) - Parsing and transformation edge cases
 - **TypeScript** (`tasks/coverage/typescript/`) - TypeScript syntax and semantics
-- **Prettier** (`crates/oxc_formatter_tests/prettier/`) - Formatting expectations (self-provisioned on first conformance run)
 
 NOTE: These suites are script-cloned and fully gitignored. ripgrep respects `.gitignore`, so a plain `rg` inside them silently returns nothing.
 Use `rg --no-ignore` (or `-u`) when searching them.

crates/oxc_formatter_core/FORMATTER_POLICY.md

@@ -110,6 +110,8 @@ node apps/oxfmt/node_modules/prettier/bin/prettier.cjs --parser <parser> --print
 
 NOTE: Prettier's default `printWidth` is `80`, but Oxfmt is `100`.
 
+Fixture tests and Prettier conformance re-format every output and record idempotency violations in their snapshots/reports.
+
 ### Fixture tests
 
 Snapshot tests driven by fixture files under `tests/fixtures/`; they cover what the Prettier conformance suite does not (suppression, divergence pins, embedded shapes, ...).
@@ -135,10 +137,14 @@ cargo test -p <crate> --test conformance
 PRETTIER_FILTER=<path> cargo test -p <crate> --test conformance -- --nocapture
 ```
 
+The Prettier suite lives under `crates/oxc_formatter_tests/prettier/` and is self-provisioned on the first conformance run. It is gitignored, so use `rg --no-ignore` (or `-u`) when searching it.
+
 JSDoc formatting is covered by plain fixture-pair tests in `oxc_formatter` (`--test jsdoc`, committed input/expected pairs — a mismatch is a failing test, not a tracked report entry).
 
 Failures must be either fixed or classified under "Known divergences".
 
-### Embedded conformance (`apps/oxfmt`)
+### E2E conformance (`apps/oxfmt`)
 
 The embedded-language features (e.g. xxx-in-js / js-in-xxx) are validated end-to-end through Oxfmt. Requires a dev build first.
+
+There are also conformance tests for each language that use real-world repositories.