feat: add AI-powered plan mode detection for auto-accept

Adds a two-stage gate before auto-accepting plan mode prompts:
1. Strict regex pre-filter - checks for numbered options + selector
2. AI confirmation - spawns Opus to classify as PLAN_MODE or NOT_PLAN_MODE

This prevents spurious Enter key presses when Claude is paused mid-thought
or experiencing network lag, rather than showing a plan approval prompt.

New files:
- src/ai-plan-checker.ts: AI checker following ai-idle-checker.ts pattern

Config fields added to RespawnConfig:
- aiPlanCheckEnabled (default: true)
- aiPlanCheckModel (default: claude-opus-4-5-20251101)
- aiPlanCheckMaxContext (default: 8000)
- aiPlanCheckTimeoutMs (default: 60000)
- aiPlanCheckCooldownMs (default: 30000)

New events: planCheckStarted, planCheckCompleted, planCheckFailed

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/types.ts

@@ -221,6 +221,16 @@ export interface RespawnConfig {
   aiIdleCheckTimeoutMs?: number;
   /** Cooldown after WORKING verdict in ms */
   aiIdleCheckCooldownMs?: number;
+  /** Whether AI plan mode check is enabled for auto-accept */
+  aiPlanCheckEnabled?: boolean;
+  /** Model to use for AI plan mode check */
+  aiPlanCheckModel?: string;
+  /** Maximum characters of terminal buffer for plan check */
+  aiPlanCheckMaxContext?: number;
+  /** Timeout for AI plan check in ms */
+  aiPlanCheckTimeoutMs?: number;
+  /** Cooldown after NOT_PLAN_MODE verdict in ms */
+  aiPlanCheckCooldownMs?: number;
 }
 
 /**
@@ -535,6 +545,16 @@ export interface PersistedRespawnConfig {
   aiIdleCheckTimeoutMs?: number;
   /** Cooldown after WORKING verdict in ms */
   aiIdleCheckCooldownMs?: number;
+  /** Whether AI plan mode check is enabled for auto-accept */
+  aiPlanCheckEnabled?: boolean;
+  /** Model to use for AI plan mode check */
+  aiPlanCheckModel?: string;
+  /** Maximum characters of terminal buffer for plan check */
+  aiPlanCheckMaxContext?: number;
+  /** Timeout for AI plan check in ms */
+  aiPlanCheckTimeoutMs?: number;
+  /** Cooldown after NOT_PLAN_MODE verdict in ms */
+  aiPlanCheckCooldownMs?: number;
   /** Duration in minutes if timed respawn was set */
   durationMinutes?: number;
 }