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/config/agents.py

@@ -11,6 +11,7 @@ AGENT_LLM_MAP: dict[str, LLMType] = {
     "coordinator": "basic",
     "planner": "basic",
     "researcher": "basic",
+    "analyst": "basic",
     "coder": "basic",
     "reporter": "basic",
     "podcast_script_writer": "basic",