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/AgentCard.tsx

@@ -112,12 +112,25 @@ export function AgentCard({ agent, onShowLogs }: AgentCardProps) {
 
         {/* Feature info */}
         <div>
-          <div className="text-xs text-muted-foreground mb-0.5">
-            Feature #{agent.featureId}
-          </div>
-          <div className="text-sm font-medium truncate" title={agent.featureName}>
-            {agent.featureName}
-          </div>
+          {agent.featureIds && agent.featureIds.length > 1 ? (
+            <>
+              <div className="text-xs text-muted-foreground mb-0.5">
+                Batch: {agent.featureIds.map(id => `#${id}`).join(', ')}
+              </div>
+              <div className="text-sm font-bold truncate">
+                Active: Feature #{agent.featureId}
+              </div>
+            </>
+          ) : (
+            <>
+              <div className="text-xs text-muted-foreground mb-0.5">
+                Feature #{agent.featureId}
+              </div>
+              <div className="text-sm font-medium truncate" title={agent.featureName}>
+                {agent.featureName}
+              </div>
+            </>
+          )}
         </div>
 
         {/* Thought bubble */}
@@ -195,7 +208,10 @@ export function AgentLogModal({ agent, logs, onClose }: AgentLogModalProps) {
                 </Badge>
               </div>
               <p className="text-sm text-muted-foreground">
-                Feature #{agent.featureId}: {agent.featureName}
+                {agent.featureIds && agent.featureIds.length > 1
+                  ? `Batch: ${agent.featureIds.map(id => `#${id}`).join(', ')}`
+                  : `Feature #${agent.featureId}: ${agent.featureName}`
+                }
               </p>
             </div>
           </div>