refactor: extract services layer, Result type, and ErrorCode classification

- Add DI container (src/services/) with AgentExecutionService, ConfigLoaderService, and ExploitationCheckerService — pure domain logic with no Temporal dependencies
- Introduce Result<T, E> type and ErrorCode enum for code-based error classification in classifyErrorForTemporal, replacing scattered string matching
- Consolidate billing/spending cap detection into utils/billing-detection.ts with shared pattern lists across message-handlers, claude-executor, and error-handling
- Extract LogStream abstraction for append-only logging with backpressure, used by both AgentLogger and WorkflowLogger
- Simplify activities.ts from inline lifecycle logic to thin wrappers delegating to services, with heartbeat and error classification
- Expand config-parser with human-readable AJV errors, security validation, and rule type-specific checks

CLAUDE.md

@@ -117,6 +117,7 @@ Defensive security tool only. Use only on systems you own or have explicit permi
 - Dense callback chains when sequential logic is clearer
 - Sacrificing readability for DRY — some repetition is fine if clearer
 - Abstractions for one-time operations
+- Backwards-compatibility shims, deprecated wrappers, or re-exports for removed code — delete the old code, don't preserve it
 
 ## Key Files