feat: add analysis step type for non-code reasoning tasks (#677) (#723)

Add a new "analysis" step type to handle reasoning and synthesis tasks
that don't require code execution, addressing the concern that routing
all non-search tasks to the coder agent was inappropriate.

Changes:
- Add ANALYSIS enum value to StepType in planner_model.py
- Create analyst_node for pure LLM reasoning without tools
- Update graph routing to route analysis steps to analyst agent
- Add analyst agent to AGENT_LLM_MAP configuration
- Create analyst prompts (English and Chinese)
- Update planner prompts with guidance on choosing between
  analysis (reasoning/synthesis) and processing (code execution)
- Change default step_type inference from "processing" to "analysis"
  when need_search=false

Co-authored-by: Willem Jiang <143703838+willem-bd@users.noreply.github.com>

src/graph/builder.py

@@ -7,6 +7,7 @@ from langgraph.graph import END, START, StateGraph
 from src.prompts.planner_model import StepType
 
 from .nodes import (
+    analyst_node,
     background_investigation_node,
     coder_node,
     coordinator_node,
@@ -39,6 +40,8 @@ def continue_to_running_research_team(state: State):
 
     if incomplete_step.step_type == StepType.RESEARCH:
         return "researcher"
+    if incomplete_step.step_type == StepType.ANALYSIS:
+        return "analyst"
     if incomplete_step.step_type == StepType.PROCESSING:
         return "coder"
     return "planner"
@@ -54,13 +57,14 @@ def _build_base_graph():
     builder.add_node("reporter", reporter_node)
     builder.add_node("research_team", research_team_node)
     builder.add_node("researcher", researcher_node)
+    builder.add_node("analyst", analyst_node)
     builder.add_node("coder", coder_node)
     builder.add_node("human_feedback", human_feedback_node)
     builder.add_edge("background_investigator", "planner")
     builder.add_conditional_edges(
         "research_team",
         continue_to_running_research_team,
-        ["planner", "researcher", "coder"],
+        ["planner", "researcher", "analyst", "coder"],
     )
     builder.add_edge("reporter", END)
     return builder