feat(#122): create LLM provider interface

Implemented abstract LLM provider interface to enable multi-provider support.

Key components:
- LlmProviderInterface: Abstract contract for all LLM providers
- LlmProviderConfig: Base configuration interface
- LlmProviderHealthStatus: Standardized health check response
- LlmProviderType: Type discriminator for runtime checks

Methods defined:
- initialize(): Async provider setup
- checkHealth(): Health status verification
- listModels(): Available model enumeration
- chat(): Synchronous completion
- chatStream(): Streaming completion (async generator)
- embed(): Embedding generation
- getConfig(): Configuration access

All methods fully documented with JSDoc.
13 tests written and passing.
Type checking verified.

Fixes #122

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/scratchpads/122-llm-provider-interface.md

@@ -0,0 +1,81 @@
+# Issue #122: Create LLM Provider Interface
+
+## Objective
+
+Define the abstract contract that all LLM providers (Ollama, Claude, OpenAI) must implement to enable multi-provider support.
+
+## Approach
+
+### Current State
+
+- `LlmService` is hardcoded to Ollama
+- Direct coupling to `ollama` npm package
+- Methods: `chat()`, `chatStream()`, `embed()`, `listModels()`, `checkHealth()`
+
+### Target Architecture
+
+```
+LlmProviderInterface (abstract)
+  ├── OllamaProvider (implements)
+  ├── ClaudeProvider (implements)
+  └── OpenAIProvider (implements)
+
+LlmManagerService
+  └── manages provider instances
+  └── routes requests to appropriate provider
+```
+
+### Interface Methods (from current LlmService)
+
+1. `chat(request)` - Synchronous chat completion
+2. `chatStream(request)` - Streaming chat completion (async generator)
+3. `embed(request)` - Generate embeddings
+4. `listModels()` - List available models
+5. `checkHealth()` - Health check
+
+### Provider Configuration
+
+Each provider needs different config:
+
+- Ollama: `{ host, timeout }`
+- Claude: `{ apiKey, baseUrl?, timeout? }`
+- OpenAI: `{ apiKey, baseUrl?, organization?, timeout? }`
+
+Need generic config interface that providers can extend.
+
+## Progress
+
+- [x] Write interface tests (TDD - RED)
+- [x] Create base types and DTOs
+- [x] Implement LlmProviderInterface
+- [x] Implement LlmProviderConfig interface
+- [x] Add JSDoc documentation
+- [x] Run tests (TDD - GREEN) - All 13 tests passed
+- [x] Type checking passed
+- [x] Refactor if needed (TDD - REFACTOR) - No refactoring needed
+
+## Testing
+
+Created a mock provider implementation to test interface contract.
+
+**Test Results:**
+
+```
+✓ src/llm/providers/llm-provider.interface.spec.ts (13 tests) 7ms
+  - initialization (2 tests)
+  - checkHealth (2 tests)
+  - listModels (2 tests)
+  - chat (2 tests)
+  - chatStream (1 test)
+  - embed (2 tests)
+  - getConfig (2 tests)
+```
+
+**Type Check:** ✅ Passed
+
+## Notes
+
+- Interface should be provider-agnostic
+- Use existing DTOs from current LlmService
+- Consider async initialization for providers
+- Health check should return standardized status