feat: add Anthropic Messages API support (#18)

Adds --llm-provider flag (openai|anthropic) to switch between API formats.

Anthropic implementation:
- POST /messages endpoint
- x-api-key + anthropic-version headers
- System prompt as top-level field (not a message)
- max_tokens: 8192 for response generation
- Parses content blocks [{type: "text", text: "..."}]

Changes:
- llm/client.go: Provider type, completeAnthropic(), doRequest() shared helper
- cmd/review-bot/main.go: --llm-provider / LLM_PROVIDER flag
- .gitea/actions/review/action.yml: llm-provider input + env
- llm/client_test.go: 4 new tests for Anthropic path

Backwards compatible — default provider is still openai.

Closes #18

.gitea/actions/review/action.yml

@@ -34,6 +34,10 @@ inputs:
   llm-model:
     description: 'LLM model name'
     required: true
+  llm-provider:
+    description: 'LLM API provider: openai or anthropic (default openai)'
+    required: false
+    default: 'openai' 
   conventions-file:
     description: 'Path to conventions file in the repo (e.g. CLAUDE.md)'
     required: false
@@ -140,6 +144,7 @@ runs:
         PATTERNS_FILES: ${{ inputs.patterns-files }}
         LLM_TEMPERATURE: ${{ inputs.temperature }}
         LLM_TIMEOUT: ${{ inputs.timeout }}
+        LLM_PROVIDER: ${{ inputs.llm-provider }}
       run: |
         ARGS=""
         if [ "${{ inputs.dry-run }}" = "true" ]; then