feat(v3-ref-seller): sync_accounts/list_accounts via AccountStore (closes #377)

Moves the sync_accounts / list_accounts implementations off the platform
class (where they were dead code — the framework never dispatched there)
onto the AccountStore returned by _make_account_store. The framework's
tool-advertising layer probes platform.accounts.upsert and
platform.accounts.list as callables; without them every sales-* agent
silently dropped both tools and the AdCP 3.0.9 §accounts/overview probe
failed.

upsert(refs, ctx) persists the buyer's AccountReference (full
billing_entity, bank and all) and returns SyncAccountsResultRow per
account. The framework projects through to_wire_sync_accounts_row,
applying the billing_entity.bank write-only strip on emit.

list(filter, ctx) returns Account[TMeta] with the billing_entity
populated from Postgres; framework's to_wire_account strips bank on
emit. Per-principal scoping by buyer-agent agent_url is enforced —
unauthenticated callers see an empty list.

Tests cover both projections end-to-end (bank persists on write, never
appears on the wire) plus a surface-check that AccountStore exposes
upsert/list callables.

Drops account_discovery from the storyboard allowlist; allowlist now
covers only the refine_products feature gap.

Author: bokelley

.github/workflows/ci.yml

@@ -683,19 +683,16 @@ jobs:
         # honest and gets pruned as upstream issues close).
         #
         # Allowlist:
-        #   - media_buy_seller/refine_products/get_products_refine — upstream
-        #     storyboard YAML bug (adcontextprotocol/adcp#702): the runner
-        #     sends 'brief' alongside buying_mode='refine', the seller
-        #     correctly rejects.
-        #   - __spec_conformance__/account_discovery/list_or_sync_accounts —
-        #     list_accounts / sync_accounts on the v3 ref seller is tracked
-        #     under #377.
+        #   - media_buy_seller/refine_products/get_products_refine — the
+        #     v3 reference seller does not implement refine_get_products();
+        #     the framework correctly returns INVALID_REQUEST for refine
+        #     requests on platforms without that optional method. Feature
+        #     gap tracked separately.
         run: |
           python -c "
           import json, sys, pathlib
           allowed = {
               ('media_buy_seller/refine_products', 'get_products_refine'),
-              ('__spec_conformance__/account_discovery', 'list_or_sync_accounts'),
           }
           p = pathlib.Path('examples/v3_reference_seller/v3-storyboard-result.json')
           with p.open() as f: