feat(M3-001): refactor ProviderService into IProviderAdapter pattern (#306)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

docs/scratchpads/m3-001-provider-adapter.md

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+# M3-001 Provider Adapter Pattern — Scratchpad
+
+## Objective
+
+Refactor ProviderService into an IProviderAdapter pattern without breaking existing Ollama flow.
+
+## Plan
+
+1. Add `IProviderAdapter` interface and supporting types to `@mosaic/types` provider package
+2. Create `apps/gateway/src/agent/adapters/` directory with:
+   - `provider-adapter.interface.ts` — IProviderAdapter + ProviderHealth + CompletionParams + CompletionEvent
+   - `ollama.adapter.ts` — extract existing Ollama logic
+3. Refactor ProviderService:
+   - Accept `IProviderAdapter[]` (injected via DI token)
+   - `registerAll()` / `listModels()` aggregates from all adapters
+   - `getAdapter(name)` — lookup by name
+   - `healthCheckAll()` — check all adapters
+   - Keep Pi ModelRegistry wiring (required by AgentService)
+4. Wire up in AgentModule
+
+## Key Findings
+
+### Pi SDK Compatibility
+
+- Pi SDK uses `ModelRegistry` as central registry; ProviderService wraps it
+- `ModelRegistry.registerProvider()` is the integration point — adapters call this
+- Pi doesn't have a native "IProviderAdapter" concept — adapters are a Mosaic abstraction on top
+- The `createAgentSession()` call in AgentService uses `modelRegistry: this.providerService.getRegistry()`
+- OllamaAdapter should call `registry.registerProvider('ollama', {...})` same as today
+- CompletionParams/CompletionEvent: Pi SDK streams via `AgentSession.prompt()`, not raw completion
+  — IProviderAdapter.createCompletion() is for future direct use; for now stub or leave as interface-only
+  — ASSUMPTION: createCompletion is reserved for future M3+ work; Pi SDK owns the actual streaming
+
+## Implementation Notes
+
+- ESM: use `.js` extensions in all imports
+- NestJS: use `@Inject()` explicitly
+- Keep RoutingService working — it only uses `providerService.listAvailableModels()`
+- Keep AgentService working — it uses `providerService.getRegistry()`, `findModel()`, `getDefaultModel()`, `listAvailableModels()`
+
+## Progress
+
+- [ ] Add types to @mosaic/types
+- [ ] Create adapters/ directory
+- [ ] Create IProviderAdapter interface file
+- [ ] Create OllamaAdapter
+- [ ] Refactor ProviderService
+- [ ] Update AgentModule
+- [ ] Run tests
+- [ ] Run quality gates
+
+## Risks
+
+- Pi SDK doesn't natively support IProviderAdapter — adapters are a layer on top
+- createCompletion() is architecturally sound but requires Pi session bypass (future work)