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>

agent.py

@@ -117,6 +117,7 @@ async def run_autonomous_agent(
     yolo_mode: bool = False,
     feature_id: Optional[int] = None,
     agent_type: Optional[str] = None,
+    testing_feature_id: Optional[int] = None,
 ) -> None:
     """
     Run the autonomous agent loop.
@@ -128,6 +129,7 @@ async def run_autonomous_agent(
         yolo_mode: If True, skip browser testing in coding agent prompts
         feature_id: If set, work only on this specific feature (used by orchestrator for coding agents)
         agent_type: Type of agent: "initializer", "coding", "testing", or None (auto-detect)
+        testing_feature_id: For testing agents, the pre-claimed feature ID to test
     """
     print("\n" + "=" * 70)
     print("  AUTONOMOUS CODING AGENT")
@@ -220,7 +222,7 @@ async def run_autonomous_agent(
         if agent_type == "initializer":
             prompt = get_initializer_prompt(project_dir)
         elif agent_type == "testing":
-            prompt = get_testing_prompt(project_dir)
+            prompt = get_testing_prompt(project_dir, testing_feature_id)
         elif feature_id:
             # Single-feature mode (used by orchestrator for coding agents)
             prompt = get_single_feature_prompt(feature_id, project_dir, yolo_mode)