feat: add multi-feature batching for coding agents

Enable the orchestrator to assign 1-3 features per coding agent subprocess,
selected via dependency chain extension + same-category fill. This reduces
cold-start overhead and leverages shared context across related features.

Orchestrator (parallel_orchestrator.py):
- Add batch tracking: _batch_features and _feature_to_primary data structures
- Add build_feature_batches() with dependency chain + category fill algorithm
- Add start_feature_batch() and _spawn_coding_agent_batch() methods
- Update _on_agent_complete() for batch cleanup across all features
- Update stop_feature() with _feature_to_primary lookup
- Update get_ready_features() to exclude all batch feature IDs
- Update main loop to build batches then spawn per available slot

CLI and agent layer:
- Add --feature-ids (comma-separated) and --batch-size CLI args
- Add feature_ids parameter to run_autonomous_agent() with batch prompt selection
- Add get_batch_feature_prompt() with sequential workflow instructions

WebSocket layer (server/websocket.py):
- Add BATCH_CODING_AGENT_START_PATTERN and BATCH_FEATURES_COMPLETE_PATTERN
- Add _handle_batch_agent_start() and _handle_batch_agent_complete() methods
- Add featureIds field to all agent_update messages
- Track current_feature_id updates as agent moves through batch

Frontend (React UI):
- Add featureIds to ActiveAgent and WSAgentUpdateMessage types
- Update KanbanColumn and DependencyGraph agent-feature maps for batch
- Update AgentCard to show "Batch: #X, #Y, #Z" with active feature highlight
- Add "Features per Agent" segmented control (1-3) in SettingsModal

Settings integration (full stack):
- Add batch_size to schemas, settings router, agent router, process manager
- Default batch_size=3, user-configurable 1-3 via settings UI
- batch_size=1 is functionally identical to pre-batching behavior

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

ui/src/components/SettingsModal.tsx

@@ -41,6 +41,12 @@ export function SettingsModal({ isOpen, onClose }: SettingsModalProps) {
     }
   }
 
+  const handleBatchSizeChange = (size: number) => {
+    if (!updateSettings.isPending) {
+      updateSettings.mutate({ batch_size: size })
+    }
+  }
+
   const models = modelsData?.models ?? []
   const isSaving = updateSettings.isPending
 
@@ -234,6 +240,30 @@ export function SettingsModal({ isOpen, onClose }: SettingsModalProps) {
               </div>
             </div>
 
+            {/* Features per Agent */}
+            <div className="space-y-2">
+              <Label className="font-medium">Features per Agent</Label>
+              <p className="text-sm text-muted-foreground">
+                Number of features assigned to each coding agent
+              </p>
+              <div className="flex rounded-lg border overflow-hidden">
+                {[1, 2, 3].map((size) => (
+                  <button
+                    key={size}
+                    onClick={() => handleBatchSizeChange(size)}
+                    disabled={isSaving}
+                    className={`flex-1 py-2 px-3 text-sm font-medium transition-colors ${
+                      (settings.batch_size ?? 1) === size
+                        ? 'bg-primary text-primary-foreground'
+                        : 'bg-background text-foreground hover:bg-muted'
+                    } ${isSaving ? 'opacity-50 cursor-not-allowed' : ''}`}
+                  >
+                    {size}
+                  </button>
+                ))}
+              </div>
+            </div>
+
             {/* Update Error */}
             {updateSettings.isError && (
               <Alert variant="destructive">