feat: enforce plan approval for multi-file modifications

The plan approval system now has enforcement at the tool execution level:
- After modifying more than 2 files, the agent MUST use plan_approval
- File-modifying tools (write, edit, delete, etc.) are blocked until
  an approved plan exists
- Clear error message guides agent to create and submit a plan

This ensures agents can't bypass plan approval instructions.

src/interfaces/AgentOptions.ts

@@ -19,6 +19,8 @@ export interface AgentOptions {
   autoApprove?: boolean;
   /** Chat mode - only read-only tools, no file modifications */
   chatMode?: boolean;
+  /** Enforce plan approval for multi-file modifications (default: true) */
+  enforcePlanApproval?: boolean;
   /** Model-specific parameters from tier detection */
   modelParams?: {
     temperature?: number;