fix: add model parameter support to base-action (#307)

- Add model field to ClaudeOptions type
- Pass ANTHROPIC_MODEL env var to runClaude function
- Handle --model argument in prepareRunConfig

This allows the model specified in action.yml to be properly passed
to the Claude CLI command.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

base-action/src/index.ts

@@ -26,6 +26,7 @@ async function run() {
       appendSystemPrompt: process.env.INPUT_APPEND_SYSTEM_PROMPT,
       claudeEnv: process.env.INPUT_CLAUDE_ENV,
       fallbackModel: process.env.INPUT_FALLBACK_MODEL,
+      model: process.env.ANTHROPIC_MODEL,
     });
   } catch (error) {
     core.setFailed(`Action failed with error: ${error}`);