feat(llm): interrogate a draft provider endpoint for its models

Once a pi-ai route became a declaration rather than a catalog lookup,
adding an OpenAI-compatible gateway meant knowing its model ids up
front. Most such endpoints publish that list at `GET /models`, but no
seam operation could ask: every one is keyed by a registered provider
route, and the provider being added has no route, no stored profile,
and no stored credential — the endpoint and key are values in a form.

Interrogation is therefore keyed by settings namespace, which a
configuration surface already holds from the configurable-provider
directory. `registerModelDiscovery` offers it per namespace,
`discoverModels` asks, and the request carries the draft itself. The
reply is candidates, not a catalog: every field but the id is optional
because most listings disclose nothing else, and adopting one is a
settings write like any other. Nothing here reads or writes settings or
credentials, so `settings.yaml` still decides what a route serves.

`llm.discoverModels` carries the same draft over the wire. Its apiKey is
the third and last payload a secret may ride, and it is never stored,
logged, or echoed; every refusal folds into `model-discovery-failed`,
naming the endpoint asked but never the credential offered.

The pi-ai side is a plain GET for OpenAI-compatible protocols only —
their listing shape is the one gateways, self-hosted servers, and the
official endpoints agree on. Others say so, sending the user to
hand-entry rather than reporting a guessed shape as an empty provider.
The reply is read under a four-megabyte ceiling held on the bytes
actually received, because the endpoint is a URL the user typed.

packages/llm/llm-pi-ai/src/index.ts

@@ -50,6 +50,7 @@ import { PiAiAdapter } from './adapter.ts'
 import { catalogProviderIds } from './catalog.ts'
 import { assertServiceable, Config, resolveProfiles } from './config.ts'
 import type { ResolvedPiAiProviderProfile } from './config.ts'
+import { discoverModels } from './discovery.ts'
 
 export { PiAiAdapter } from './adapter.ts'
 export type { PiAiAdapterOptions } from './adapter.ts'
@@ -176,6 +177,10 @@ export function apply(ctx: Context, config: Config): void {
     directoryFacts = entries
   }
   ensureDirectory()
+  // Interrogating an endpoint is a configuration-time action over a draft, so
+  // it is offered for the whole namespace rather than per route: the provider
+  // a surface is adding does not exist yet.
+  ctx.llm.registerModelDiscovery(NS, discoverModels)
   // Route effects bind to this apply fiber via the stable `ctx` reference,
   // even when a swap runs inside the scoped settings callback below. A bare
   // mount (zero routes) is the dormant posture: nothing registers until a