fix(models): list providers that extensions register at session_start

[SoulKyu]

Summary

Models of pi-claude-bridge showed in the picker only on the first listing after Pi Web started. They vanished as soon as the user switched folders, and a session could not start on them.

pi-web builds a fresh ModelRuntime for every model listing (createAgentSessionServices() in /api/models, /api/models/default, /api/models/enabled) and never starts a session in it. The bridge registers its provider eagerly only in the first runtime of a process. Later instances defer to session_start, so that a subagent sharing its parent's registry does not overwrite the parent's stream function. In pi-web, every runtime after the first therefore lacked the provider:

runtime 1: zai,claude-bridge
runtime 2: zai
runtime 3: zai

The same deferral breaks other hosts; it is tracked upstream in elidickinson/pi-claude-bridge#149. This PR makes Pi Web tolerate any extension that registers its provider at session_start.

Listings

• lib/deferred-provider-models.ts remembers the available models of every runtime Pi Web builds: listings, createModelRuntimeWithExtensions(), and sessions once their extensions are bound.
• withDeferredProviderModels() adds back the models of providers a runtime does not register at all. A provider it registered but cannot use (signed out, no key) is never added back, so auth state stays truthful.
• When a new provider is remembered, the models cache is invalidated. Otherwise a listing cached before it would hide it for the cache lifetime.

Sessions

• startRpcSession() builds a session whose requested model (new chat) or restored model (resumed chat) belongs to such a deferred provider on the default model. Once the extensions are bound and session_start has run, it switches to that model with set_model. Before, a new chat on a bridge model failed the scope check, and a resumed one silently fell back to the default.

Known limit: a project-only extension that defers its provider would also appear in other folders' listings. Picking it there leaves the session on its default model.

docs/agents/models.md gets a section on this, and the AGENTS.md file map is updated.

Test plan

• tsc --noEmit, npm run lint, npm run build
• npm test: 2286/2286. The new lib/deferred-provider-models.test.mjs covers:
  • a later runtime lists the remembered models, with and without a provider filter;
  • a registered but signed-out provider is not added back;
  • a deferred model is found only while the runtime lacks its provider.
• Against a production build on a loopback port, with pi-claude-bridge 0.9.1 installed globally:
  • /api/models for folders A, B, C, then A again: every listing has claude-bridge (13 models) and zai (7). Before, only the first did;
  • POST /api/agent/new with claude-bridge/claude-haiku-4-5 in a folder that was not listed first: the session starts on Haiku and the reply comes from the bridge;
  • server restart, then reopening that session: get_state reports claude-bridge/claude-haiku-4-5, not the default model.