Add compatibility guidance to AGENTS.md

AGENTS.md

@@ -33,6 +33,7 @@ Before touching a subsystem, read the relevant notes in `contributing/`: `ARCHIT
 - Don't use function-level (inner) imports to break circular imports. Inject the dependency or move the shared code to a lower-level module instead.
 - Never edit a migration that has already been applied or released; add a new migration instead.
 - Derive paths under `SERVER_DIR_PATH` on access (a `get_*` function), not as module-level constants, so that patching `settings.SERVER_DIR_PATH` redirects all of them. Tests rely on this to keep server state out of the real `~/.dstack`.
+- Preserve client/server backward compatibility when updating Pydantic models. New fields must be excluded from client requests when unset, for compatibility with older servers (see `core/compatibility/*.py`). If the allowed domain/type of an existing field is extended, server responses may need to be patched for older clients (see `server/compatibility/*.py`). No need to exclude new fields from server responses, since clients rely on `validate_extra_ignore`.
 
 ## Testing Guidelines
 - Default to `uv run pytest`. Use markers from `src/tests/conftest.py` like `--runpostgres` if need to include specific tests.