refactor: clean up agent architecture and coordination loop

Squash merged refactoring of agent architecture.

core/planning_agent.py

@@ -0,0 +1,13 @@
+import logging
+from core.agent import BaseAgent
+from .coding_prompt import CODING_AGENT_SYSTEM_PROMPT
+
+logger: logging.Logger = logging.getLogger("agent-planning")
+
+
+class PlanningAgent(BaseAgent):
+    """AI agent that analyzes a PR/issue and builds an implementation plan."""
+
+    def __init__(self, model_name: str) -> None:
+        super().__init__(model_name)
+        self.system_prompt = CODING_AGENT_SYSTEM_PROMPT