Key design decisions:
- ID-JAG is an IETF draft spec (draft-ietf-oauth-identity-chaining), so it lives under `authlib/oauth2/drafts/` rather than alongside the finalized RFC 7523 module. This mirrors the existing `authlib/jose/drafts/` convention and keeps RFC-named packages reserved for published RFCs.
- Reuses the same `grant_type` URI as RFC 7523 (jwt-bearer); differentiation is via the `typ: oauth-id-jag+jwt` JWT header.
- Extends JWTBearerGrant with stricter claims (sub, iat, jti, client_id all required) and new hooks for IdP key resolution and policy checks.
- Cannot coexist with JWTBearerGrant on the same authorization server.
- Uses joserfc for all JWT operations per Authlib 1.0+ standards.
- Refresh tokens are never issued (per ID-JAG spec requirement).

New files:
- authlib/oauth2/drafts/__init__.py            — drafts package, re-exports
- authlib/oauth2/drafts/id_jag.py              — IDJAGGrant implementation
- tests/core/test_oauth2/test_id_jag.py        — unit tests (33)
- tests/flask/test_oauth2/test_id_jag_grant.py — Flask integration (12)
- docs/oauth2/specs/id-jag.rst                 — documentation

Modified files:
- docs/oauth2/specs/index.rst                  — add doc reference
- AGENTS.md                                    — document drafts/ convention

Public import: `from authlib.oauth2.drafts import IDJAGGrant`

Addresses reviewer feedback on PR for authlib#870:

Addressed reviewer's comment. ID-JAG shared only the `grant_type` URI and a handful of small helpers with JWTBearerGrant; inheriting forced three RFC 7523 hooks (resolve_issuer_client, resolve_client_public_key, has_granted_permission) onto the class that had no meaning in the ID-JAG three-party model and were stubbed with NotImplementedError.

Changes:

- IDJAGGrant now inherits directly from BaseGrant + TokenEndpointMixin.
- Removed the three dead NotImplementedError stubs entirely (no longer present on the class, not merely overridden).
- Inlined the small JWT pipeline previously inherited from JWTBearerGrant: _extract_assertion, _verify_claims, create_token_response (~30 LOC total).  These are now `_`-prefixed to mark them as internal implementation, since they are no longer part of an inherited public contract.
- Added LEEWAY = 60 constant (was inherited).
- Updated module + class docstrings to explain the deliberate non-inheritance and to point readers at the right hooks.

Tests:

- Removed the three tests that asserted NotImplementedError on the now-deleted stubs.
- Added two structural tests that pin the new design:
  * test_does_not_inherit_from_jwt_bearer_grant
  * test_does_not_expose_rfc7523_hooks

Docs & metadata:

- docs/oauth2/specs/id-jag.rst: new "Using IDJAGGrant" paragraph clarifies that the class is standalone, not a JWTBearerGrant subclass.

Compatibility:

- Public API surface unchanged: from authlib.oauth2.drafts import IDJAGGrant, all six application hooks, sign() static helper, GRANT_TYPE / REQUIRED_TYP / CLAIMS_OPTIONS / LEEWAY constants.
- Application subclasses that only implement the documented ID-JAG hooks are unaffected.
- Subclasses that accidentally relied on the inherited RFC 7523 hooks (resolve_issuer_client / resolve_client_public_key / has_granted_permission) will now get AttributeError instead of NotImplementedError — surfacing the misuse earlier and louder.

Test results: 123 passed (32 ID-JAG core + 79 RFC 7523-area + 12 Flask integration); zero regressions.

…jag.py (97.4%): Missing lines 250-251,309

…get_audiences

Addresses three reviewer comments on the ID-JAG PR:

> "You need to catch the joserfc errors and re-throw an authlib OAuth2Error so that the server won't return a 500."
>
> - line 244-247: may raise a joserfc DecodeError
> - line 270-274: `deprecate` is used when we will deprecate something; this code is not released yet, there is nothing to deprecate
> - line 306-309: if empty list is not allowed, it is better to raise NotImplementedError

Changes to authlib/oauth2/drafts/id_jag.py:

- `_extract_assertion`: wrap `jws.extract_compact` in a `try/except (JoseError, ValueError)` block and re-raise as `InvalidGrantError("Invalid JWT assertion")`.  Malformed compact JWTs previously bubbled a joserfc `DecodeError` up to the framework, producing an HTTP 500 instead of the RFC 6749 5.2 `invalid_grant` response.
- `_verify_claims`: removed the `deprecate()` call and the conditional branch that skipped audience validation when `get_audiences()` returned `[]`.  ID-JAG is a new, unreleased grant, so there is no prior behaviour to deprecate; `aud` validation is now unconditional and mandatory.
- `get_audiences`: default implementation now `raise NotImplementedError()` in line with the other application hooks (`resolve_issuer_key`, `resolve_client_by_id`, `authenticate_user`, `check_jti`, `check_id_jag_permission`).  Application subclasses must override it.
- Removed the now-unused `from authlib.deprecate import deprecate` import.
- Class docstring: merged `get_audiences` into the single "must implement" list; removed the special-case "should implement" section.
- Hooks-section comment simplified - no more special-case narrative for `get_audiences`.

Changes to tests/core/test_oauth2/test_id_jag.py:

- Renamed `test_base_get_audiences_returns_empty_list` to `test_base_get_audiences_raises_not_implemented` and updated the assertion.
- Added `test_extract_assertion_rejects_malformed_compact`: feeds `"not.a.jwt"` into the grant and asserts `InvalidGrantError`, guarding against joserfc `DecodeError` regressing to a 500.

Test results: 126 passed (35 ID-JAG core + 12 Flask integration + 79 other RFC 7523 / oauth2 core), zero regressions, 100% line + branch coverage on `authlib/oauth2/drafts/`.