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>

server/routers/settings.py

@@ -92,6 +92,7 @@ async def get_settings():
         ollama_mode=_is_ollama_mode(),
         testing_agent_ratio=_parse_int(all_settings.get("testing_agent_ratio"), 1),
         playwright_headless=_parse_bool(all_settings.get("playwright_headless"), default=True),
+        batch_size=_parse_int(all_settings.get("batch_size"), 3),
     )
 
 
@@ -110,6 +111,9 @@ async def update_settings(update: SettingsUpdate):
     if update.playwright_headless is not None:
         set_setting("playwright_headless", "true" if update.playwright_headless else "false")
 
+    if update.batch_size is not None:
+        set_setting("batch_size", str(update.batch_size))
+
     # Return updated settings
     all_settings = get_all_settings()
     return SettingsResponse(
@@ -119,4 +123,5 @@ async def update_settings(update: SettingsUpdate):
         ollama_mode=_is_ollama_mode(),
         testing_agent_ratio=_parse_int(all_settings.get("testing_agent_ratio"), 1),
         playwright_headless=_parse_bool(all_settings.get("playwright_headless"), default=True),
+        batch_size=_parse_int(all_settings.get("batch_size"), 3),
     )