feat: add three-tier model system with Bedrock support

Introduce small/medium/large model tiers so agents use the appropriate
model for their task complexity. Pre-recon uses Opus (large) for deep
source code analysis, most agents use Sonnet (medium), and report uses
Haiku (small) for summarization.

- Add src/ai/models.ts with ModelTier type and resolveModel()
- Add modelTier field to AgentDefinition
- Refactor claude-executor env var passthrough into loop
- Add Bedrock credential validation in preflight and CLI
- Pass through Bedrock and model env vars in docker-compose

src/types/agents.ts

@@ -58,6 +58,7 @@ export interface AgentDefinition {
   prerequisites: AgentName[];
   promptTemplate: string;
   deliverableFilename: string;
+  modelTier?: 'small' | 'medium' | 'large';
 }
 
 /**