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>

autonomous_agent_demo.py

@@ -141,6 +141,13 @@ Authentication:
         help="Agent type (used by orchestrator to spawn specialized subprocesses)",
     )
 
+    parser.add_argument(
+        "--testing-feature-id",
+        type=int,
+        default=None,
+        help="Feature ID to regression test (used by orchestrator for testing agents)",
+    )
+
     # Testing agent configuration
     parser.add_argument(
         "--testing-ratio",
@@ -197,6 +204,7 @@ def main() -> None:
                     yolo_mode=args.yolo,
                     feature_id=args.feature_id,
                     agent_type=args.agent_type,
+                    testing_feature_id=args.testing_feature_id,
                 )
             )
         else: