fix: stream memory workers through composed custom APIs

[SavvasMohito]

Summary

• Fixes Bug: background consolidation crashes Pi with custom provider API #30: observer/reflector/dropper no longer hard-wire @earendil-works/pi-ai/compat streamSimple, which cannot dispatch pi.registerProvider APIs (cursor-sdk, CLIProxyAPI, commandcode, …).
• Workers resolve a stream function in this order: explicit override, ModelRegistry.streamSimple when Pi exposes it (earendil-works/pi#8964), then getRegisteredProviderConfig(id).streamSimple matching model.api, then compat fallback.
• Custom-provider-only setups can leave observational-memory.model unset. A second built-in provider (OpenAI/OpenRouter) is optional, not required.
• Registry lookup is try/caught so incomplete host/test doubles still use compat (OAuth e2e stays green).

Test plan

• npm test (263 tests)
• npm run typecheck
• Fresh Pi: install this branch (local path or git:github.com/SavvasMohito/pi-observational-memory@fix/composed-custom-api-stream)
• Session model whose api comes from pi.registerProvider (not a built-in): no PI_OBSERVATIONAL_MEMORY_PASSIVE, no observational-memory.model override onto OpenAI/OpenRouter/etc. One successful prompt, exit 0, process stays up, no No API provider registered for api: <custom-api>
• Interactive session: [redacted] turns, /om:status without process crash

[SavvasMohito]

Confirmed on a live Pi 0.84.4 session after installing this branch (local checkout of fix/composed-custom-api-stream).

Setup:

• Session model from pi.registerProvider (cursor-sdk), no observational-memory.model override
• PI_OBSERVATIONAL_MEMORY_PASSIVE unset
• No second built-in worker provider

Result:

• Interactive turns completed; process stayed up
• /om:status worked
• No No API provider registered for api: cursor-sdk after a successful turn