Inject current date into deep research planning and query prompts (#1347)

Deep research generated search queries from the LLM's training-cutoff
knowledge, so it emitted stale-year queries like "best Python tutorials
2025" when the actual year is later (issue #1341). The chat/agent path
already grounds the model with "Today is ..." (src/agent_loop.py); the
deep research planning and query-generation prompts had no equivalent.

Add a small current_date_context() helper and prepend it at the plan and
query-generation prompt sites (and the research_handler plan preview path
that reuses RESEARCH_PLAN_PROMPT). System-TZ local, portable strftime.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/research_handler.py

@@ -161,10 +161,10 @@ class ResearchHandler:
     ) -> Optional[dict]:
         """Generate a research plan for user review before starting research."""
         try:
-            from src.deep_research import RESEARCH_PLAN_PROMPT
+            from src.deep_research import RESEARCH_PLAN_PROMPT, current_date_context
             from src.llm_core import llm_call_async
 
-            prompt = RESEARCH_PLAN_PROMPT.format(question=query)
+            prompt = current_date_context() + RESEARCH_PLAN_PROMPT.format(question=query)
             response = await llm_call_async(
                 url=llm_endpoint,
                 model=llm_model,