refactor: orchestrator pre-selects features for all agents

Replace agent-initiated feature selection with orchestrator pre-selection
for both coding and testing agents. This ensures Mission Control displays
correct feature numbers for testing agents (previously showed "Feature #0").

Key changes:

MCP Server (mcp_server/feature_mcp.py):
- Add feature_get_by_id tool for agents to fetch assigned feature details
- Remove obsolete tools: feature_get_next, feature_claim_next,
  feature_claim_for_testing, feature_get_for_regression
- Remove helper functions and unused imports (text, OperationalError, func)

Orchestrator (parallel_orchestrator.py):
- Change running_testing_agents from list to dict[int, Popen]
- Add claim_feature_for_testing() with random selection
- Add release_testing_claim() method
- Pass --testing-feature-id to spawned testing agents
- Use unified [Feature #X] output format for both agent types

Agent Entry Points:
- autonomous_agent_demo.py: Add --testing-feature-id CLI argument
- agent.py: Pass testing_feature_id to get_testing_prompt()

Prompt Templates:
- coding_prompt.template.md: Update to use feature_get_by_id
- testing_prompt.template.md: Update workflow for pre-assigned features
- prompts.py: Update pre-claimed headers for both agent types

WebSocket (server/websocket.py):
- Simplify tracking with unified [Feature #X] pattern
- Remove testing-specific parsing code

Assistant (server/services/assistant_chat_session.py):
- Update help text with current available tools

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

client.py

@@ -54,9 +54,7 @@ def get_playwright_headless() -> bool:
 FEATURE_MCP_TOOLS = [
     # Core feature operations
     "mcp__features__feature_get_stats",
-    "mcp__features__feature_get_next",
-    "mcp__features__feature_claim_next",  # Atomic get+claim for parallel execution
-    "mcp__features__feature_get_for_regression",
+    "mcp__features__feature_get_by_id",  # Get assigned feature details
     "mcp__features__feature_mark_in_progress",
     "mcp__features__feature_mark_passing",
     "mcp__features__feature_mark_failing",  # Mark regression detected
@@ -64,11 +62,12 @@ FEATURE_MCP_TOOLS = [
     "mcp__features__feature_create_bulk",
     "mcp__features__feature_create",
     "mcp__features__feature_clear_in_progress",
+    "mcp__features__feature_release_testing",  # Release testing claim
     # Dependency management
     "mcp__features__feature_add_dependency",
     "mcp__features__feature_remove_dependency",
     "mcp__features__feature_set_dependencies",
-    # Parallel execution support
+    # Query tools
     "mcp__features__feature_get_ready",
     "mcp__features__feature_get_blocked",
     "mcp__features__feature_get_graph",