fix(ui-settings-models): address review — derived key fact, required render seat, spec sync

The provider-card seat's keyConfigured now derives from the reference the
page would use — the profile's apiKeyEnv, or the page's derived
<ROUTE>_API_KEY while the profile names none — so the add-provider draft
agrees with its own editor about an existing conventional credential (the
store joins the derived describe in the same batched call, as
ProviderRow.derivedCredential). ModelsSectionProps makes the renderSlot seat
required so a direct render that forgets it fails to compile; the one such
render in provider-form.client.spec regained a real mount and the test
boilerplate collapsed to renderSlot={() => null}. The extension-slots Agent
Note now states the keyed cell's real override rule (same priority throws,
a different priority shadows), and docs/subsystems/slots.md carries the two
new seats in its hierarchy, both languages.

docs/subsystems/slots.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/subsystems/slots.md
-slots.md: 6eb61780ca2f06ebc38a0fcf2638a7fafcd5ceee
-slots.zh.md: e5a25763d382c228525f3da24694e41dd09737e0
+slots.md: 8e115e30aed68e543eca2f1aac6e28ad9f57cf73
+slots.zh.md: 3894b69d8d020b4bb67ce325d389ab3f20cfcc9a

docs/subsystems/slots.md

@@ -123,6 +123,8 @@ root
 │     ├─ settings.onboarding
 │     └─ settings.section
 │        ├─ settings.general.item
+│        ├─ settings.models.provider-card
+│        ├─ settings.models.footer
 │        └─ settings.plugins.tab
 │           └─ settings.plugin.item
 ├─ conversation

docs/subsystems/slots.zh.md

@@ -123,6 +123,8 @@ root
 │     ├─ settings.onboarding
 │     └─ settings.section
 │        ├─ settings.general.item
+│        ├─ settings.models.provider-card
+│        ├─ settings.models.footer
 │        └─ settings.plugins.tab
 │           └─ settings.plugin.item
 ├─ conversation