docs(semantic): add AGENTS.md test guidance for agents (#23441)

This PR adds an `AGENTS.md` to `crates/oxc_semantic`. There are a range of potential locations for tests to go here, and AI agents often get it wrong. `AGENTS.md` not explicitly lays out where the test case should go to avoid confusion.

crates/oxc_semantic/AGENTS.md

@@ -0,0 +1,63 @@
+# Coding agent guides for `crates/oxc_semantic`
+
+## Testing semantic behavior
+
+Choose the test location based on the behavior being protected. Do not add duplicate coverage if the case is already covered by `just coverage`, an existing semantic fixture, or an upstream-derived fixture.
+
+For OXC-specific scope, symbol, binding, or reference regressions, add a snapshot fixture under:
+
+```text
+crates/oxc_semantic/tests/fixtures/oxc/<area>/<case>.<ext>
+```
+
+The semantic snapshot harness records the scope tree, symbols, and references for every `.{js,jsx,ts,tsx}` fixture. Use a short descriptive case name, or an issue-based name when the fixture fixes a tracked issue.
+
+Run semantic snapshot tests with:
+
+```sh
+cargo test -p oxc_semantic --test main
+```
+
+Review intentional snapshot changes with:
+
+```sh
+cargo insta review -p oxc_semantic
+```
+
+## Integration tests
+
+Use `crates/oxc_semantic/tests/integration/` when the regression is best asserted through the Rust API rather than a snapshot. This is usually right for exact enum values, module data, CFG shape, class table behavior, or direct `Scoping` API expectations.
+
+Run integration tests with:
+
+```sh
+cargo test -p oxc_semantic --test integration
+```
+
+For CFG integration tests, enable the `cfg` feature:
+
+```sh
+cargo test -p oxc_semantic --test integration --features cfg cfg
+```
+
+## Conformance checks
+
+Use `crates/oxc_semantic/tests/conformance/` for broad invariants that should be checked against every semantic fixture, such as reflexive symbol declarations or identifier reference wiring.
+
+Do not add one-off bug regressions there unless the check is intentionally generalized across all fixtures.
+
+## Coverage fixtures
+
+Use `tasks/coverage/misc/pass/` or `tasks/coverage/misc/fail/` only when the bug is best represented as a parser/semantic coverage case, especially for diagnostics surfaced by `cargo coverage semantic`.
+
+Run a focused coverage case with:
+
+```sh
+cargo coverage semantic --filter misc/fail/<case>.<ext>
+```
+
+Run all semantic coverage with:
+
+```sh
+cargo coverage semantic
+```