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/config/execution-limits.ts

@@ -0,0 +1,93 @@
+/**
+ * @fileoverview Centralized limits for the Execution Bridge system.
+ *
+ * These constants define resource limits for parallel task execution,
+ * group scheduling, and model selection.
+ *
+ * @module config/execution-limits
+ */
+
+// ============================================================================
+// Parallel Execution Limits
+// ============================================================================
+
+/**
+ * Maximum tasks to execute in parallel within a single group.
+ * Higher values increase throughput but may strain system resources.
+ */
+export const MAX_PARALLEL_TASKS_PER_GROUP = 5;
+
+/**
+ * Default timeout for an execution group in milliseconds (30 minutes).
+ * If all tasks in a group don't complete within this time, stragglers are cancelled.
+ */
+export const GROUP_TIMEOUT_MS = 30 * 60 * 1000;
+
+/**
+ * Maximum retry attempts for a failed task before marking as permanently failed.
+ */
+export const MAX_TASK_RETRIES = 2;
+
+/**
+ * Delay between task retries in milliseconds (10 seconds).
+ */
+export const TASK_RETRY_DELAY_MS = 10 * 1000;
+
+// ============================================================================
+// Model Selection Limits
+// ============================================================================
+
+/**
+ * Default model when no recommendation is provided.
+ */
+export const DEFAULT_MODEL: 'opus' | 'sonnet' | 'haiku' = 'sonnet';
+
+/**
+ * Token threshold for switching execution modes.
+ * Tasks with estimated tokens below this use task-tool mode.
+ * Tasks with estimated tokens above this use session mode.
+ */
+export const TOKEN_THRESHOLD_FOR_SESSION_MODE = 50000;
+
+/**
+ * Token threshold for low-complexity tasks (haiku-appropriate).
+ */
+export const TOKEN_THRESHOLD_HAIKU = 15000;
+
+// ============================================================================
+// Context Management Limits
+// ============================================================================
+
+/**
+ * Delay between /clear and /init commands in milliseconds.
+ */
+export const CONTEXT_REFRESH_DELAY_MS = 2000;
+
+/**
+ * Maximum pending context refresh operations.
+ */
+export const MAX_PENDING_CONTEXT_REFRESHES = 10;
+
+// ============================================================================
+// Execution Bridge Limits
+// ============================================================================
+
+/**
+ * Maximum execution groups to track in history.
+ */
+export const MAX_EXECUTION_HISTORY = 50;
+
+/**
+ * Polling interval for execution progress in milliseconds.
+ */
+export const EXECUTION_POLL_INTERVAL_MS = 1000;
+
+/**
+ * Maximum total tasks in a single execution plan.
+ */
+export const MAX_TASKS_PER_PLAN = 100;
+
+/**
+ * Grace period after group completion before cleanup in milliseconds.
+ */
+export const GROUP_CLEANUP_DELAY_MS = 5000;