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/agent.py

@@ -17,11 +17,11 @@ from ..utils.project_helpers import get_project_path as _get_project_path
 from ..utils.validation import validate_project_name
 
 
-def _get_settings_defaults() -> tuple[bool, str, int, bool]:
+def _get_settings_defaults() -> tuple[bool, str, int, bool, int]:
     """Get defaults from global settings.
 
     Returns:
-        Tuple of (yolo_mode, model, testing_agent_ratio, playwright_headless)
+        Tuple of (yolo_mode, model, testing_agent_ratio, playwright_headless, batch_size)
     """
     import sys
     root = Path(__file__).parent.parent.parent
@@ -42,7 +42,12 @@ def _get_settings_defaults() -> tuple[bool, str, int, bool]:
 
     playwright_headless = (settings.get("playwright_headless") or "true").lower() == "true"
 
-    return yolo_mode, model, testing_agent_ratio, playwright_headless
+    try:
+        batch_size = int(settings.get("batch_size", "3"))
+    except (ValueError, TypeError):
+        batch_size = 3
+
+    return yolo_mode, model, testing_agent_ratio, playwright_headless, batch_size
 
 
 router = APIRouter(prefix="/api/projects/{project_name}/agent", tags=["agent"])
@@ -91,19 +96,22 @@ async def start_agent(
     manager = get_project_manager(project_name)
 
     # Get defaults from global settings if not provided in request
-    default_yolo, default_model, default_testing_ratio, playwright_headless = _get_settings_defaults()
+    default_yolo, default_model, default_testing_ratio, playwright_headless, default_batch_size = _get_settings_defaults()
 
     yolo_mode = request.yolo_mode if request.yolo_mode is not None else default_yolo
     model = request.model if request.model else default_model
     max_concurrency = request.max_concurrency or 1
     testing_agent_ratio = request.testing_agent_ratio if request.testing_agent_ratio is not None else default_testing_ratio
 
+    batch_size = default_batch_size
+
     success, message = await manager.start(
         yolo_mode=yolo_mode,
         model=model,
         max_concurrency=max_concurrency,
         testing_agent_ratio=testing_agent_ratio,
         playwright_headless=playwright_headless,
+        batch_size=batch_size,
     )
 
     # Notify scheduler of manual start (to prevent auto-stop during scheduled window)