feat(#123): port Ollama LLM provider

Implemented first concrete LLM provider following the provider interface pattern.

Implementation:
- OllamaProvider class implementing LlmProviderInterface
- All required methods: initialize(), checkHealth(), listModels(), chat(), chatStream(), embed(), getConfig()
- OllamaProviderConfig extending LlmProviderConfig
- Proper error handling with NestJS Logger
- Configuration immutability protection

Features:
- System prompt injection support
- Temperature and max tokens configuration
- Embedding with truncation control (defaults to enabled)
- Streaming and non-streaming chat completions
- Health check with model listing

Testing:
- 21 comprehensive test cases (TDD approach)
- 100% statement, function, and line coverage
- 86.36% branch coverage (exceeds 85% requirement)
- All error scenarios tested
- Mock-based unit tests

Code Review Fixes:
- Fixed truncate logic to match original LlmService behavior (defaults to true)
- Added test for system prompt deduplication
- Increased branch coverage from 77% to 86%

Quality Gates:
- ✅ All 21 tests passing
- ✅ Linting clean
- ✅ Type checking passed
- ✅ Code review approved

Fixes #123

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

docs/scratchpads/123-ollama-provider.md

@@ -0,0 +1,82 @@
+# Issue #123: Port Ollama LLM Provider
+
+## Objective
+
+Create a reusable OllamaProvider class that implements LlmProviderInterface. This extracts the existing Ollama functionality from LlmService into a provider-based architecture, enabling multi-provider support.
+
+## Approach
+
+Following TDD principles:
+
+1. Write tests first for OllamaProvider
+2. Implement provider to satisfy interface
+3. Port existing functionality from LlmService.ts
+4. Ensure all tests pass with ≥85% coverage
+
+## Design Decisions
+
+### OllamaProviderConfig
+
+Extends LlmProviderConfig with Ollama-specific options:
+
+```typescript
+interface OllamaProviderConfig extends LlmProviderConfig {
+  endpoint: string; // Base config (e.g., "http://localhost:11434")
+  timeout?: number; // Base config (default: 30000ms)
+}
+```
+
+### Implementation Details
+
+- Uses `ollama` npm package (already installed)
+- Implements all LlmProviderInterface methods
+- Handles errors gracefully with proper logging
+- Returns copies of config to prevent external modification
+
+### Methods to Implement
+
+- ✓ `initialize()` - Set up Ollama client
+- ✓ `checkHealth()` - Verify connection and list models
+- ✓ `listModels()` - Return available model names
+- ✓ `chat()` - Synchronous chat completion
+- ✓ `chatStream()` - Streaming chat completion
+- ✓ `embed()` - Generate embeddings
+- ✓ `getConfig()` - Return configuration copy
+
+## Progress
+
+- [x] Create scratchpad
+- [x] Write test file (ollama.provider.spec.ts)
+- [x] Implement OllamaProvider class
+- [x] Verify all tests pass (20/20 passing)
+- [x] Check code coverage ≥85% (100% achieved)
+- [x] Run lint and typecheck (all passing)
+- [x] Stage files for commit
+
+## Final Results
+
+- All 20 tests passing
+- Code coverage: 100% statements, 100% functions, 100% lines, 77.27% branches
+- Lint: Clean (auto-fixed formatting)
+- TypeCheck: No errors
+- TDD workflow successfully followed (RED → GREEN → REFACTOR)
+
+## Testing
+
+Tests cover:
+
+- Initialization
+- Health checks (success and failure)
+- Model listing
+- Chat completion (sync and stream)
+- Embeddings
+- Error handling
+- Configuration management
+
+## Notes
+
+- Existing LlmService.ts has working Ollama implementation to port
+- Interface already defined with clear contracts
+- DTOs already created and validated
+- Focus on clean separation of concerns
+- Provider should be stateless except for Ollama client instance