feat(execution): add ExecutionBridge for parallel task execution

Implements the Execution Optimizer Rework plan that enables the system
to actually use the optimizer metadata (parallelGroups, agentType,
recommendedModel, requiresFreshContext, estimatedTokens) that was
previously generated but ignored.

New components:
- ExecutionBridge: Central coordinator that loads optimized plans,
  manages parallel execution within groups, and coordinates with
  SpawnOrchestrator for session-based execution
- ModelSelector: Routes tasks to appropriate models (opus/sonnet/haiku)
  based on user defaults and agent type overrides. Optimizer
  recommendations are advisory only - user preferences always win
- GroupScheduler: Builds topologically ordered execution groups,
  manages dependencies, determines execution mode (session vs task-tool)
- ContextManager: Handles fresh context requirements via /clear+/init
  or new session spawning

Features:
- Parallel task execution within groups (configurable limit)
- Group-level dependency tracking (lower groups complete first)
- Partial failure handling (continue with non-dependent tasks)
- Model configuration in App Settings > Models tab
- Agent type overrides (explore, implement, test, review)
- Execution control API endpoints (start, pause, resume, cancel)
- SSE events for real-time execution progress visibility
- Execution history tracking

API endpoints:
- GET/POST/PUT /api/execution/* for execution control
- GET/PUT /api/execution/model-config for model settings

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

src/types.ts

@@ -139,6 +139,8 @@ export interface SessionState {
   autoCompactThreshold?: number;
   /** Auto-compact prompt */
   autoCompactPrompt?: string;
+  /** Image watcher enabled for this session */
+  imageWatcherEnabled?: boolean;
   /** Total cost in USD */
   totalCost?: number;
   /** Input tokens used */
@@ -1270,3 +1272,38 @@ export type {
   AgentContext,
   SpawnPersistedState,
 } from './spawn-types.js';
+
+// ========== Execution Bridge Re-exports ==========
+
+export type {
+  ExecutionStatus,
+  ExecutionProgress,
+  TaskAssignment as ExecutionTaskAssignment,
+  PlanItem,
+  ExecutionHistoryEntry,
+} from './execution-bridge.js';
+
+export type {
+  ModelTier,
+  AgentType,
+  ExecutionMode,
+  ModelConfig,
+  ModelSelection,
+  ExecutionModeSelection,
+  TaskCharacteristics,
+} from './model-selector.js';
+
+export type {
+  GroupTaskStatus,
+  ExecutionGroupStatus,
+  GroupTask,
+  ExecutionGroup,
+  ExecutionSchedule,
+} from './group-scheduler.js';
+
+export type {
+  ContextRefreshMethod,
+  ContextRefreshStatus,
+  ContextRefreshRequest,
+  ContextRefreshResult,
+} from './context-manager.js';