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/lib/types.ts

@@ -199,7 +199,8 @@ export interface ActiveAgent {
   agentIndex: number  // -1 for synthetic completions
   agentName: AgentMascot | 'Unknown'
   agentType: AgentType  // "coding" or "testing"
-  featureId: number
+  featureId: number        // Current/primary feature (backward compat)
+  featureIds: number[]     // All features in batch
   featureName: string
   state: AgentState
   thought?: string
@@ -270,6 +271,7 @@ export interface WSAgentUpdateMessage {
   agentName: AgentMascot | 'Unknown'
   agentType: AgentType  // "coding" or "testing"
   featureId: number
+  featureIds?: number[]  // All features in batch (may be absent for backward compat)
   featureName: string
   state: AgentState
   thought?: string
@@ -530,6 +532,7 @@ export interface Settings {
   ollama_mode: boolean
   testing_agent_ratio: number  // Regression testing agents (0-3)
   playwright_headless: boolean
+  batch_size: number  // Features per coding agent batch (1-3)
 }
 
 export interface SettingsUpdate {
@@ -537,6 +540,7 @@ export interface SettingsUpdate {
   model?: string
   testing_agent_ratio?: number
   playwright_headless?: boolean
+  batch_size?: number
 }
 
 export interface ProjectSettingsUpdate {