feat: inject today's date into system prompt and add extended thinking support

Fixes age validation errors caused by the LLM not knowing the current date.

Changes:
- prompts.py: inject date.today() at the top of both system prompts so the
  LLM can accurately calculate a child's age from their date of birth
- llm.py: add optional thinking_budget parameter to complete(); when set,
  passes thinking={"type": "enabled", "budget_tokens": N} to litellm and
  raises max_tokens to thinking_budget + 4096 (Anthropic models only)
- config.py: add thinking_budget field, read from THINKING_BUDGET env var
- .env.example: document the THINKING_BUDGET option
- core.py: pass thinking_budget through to llm.complete()
- main.py: pass thinking_budget when constructing EmailAgent
- chat_app.py: switch from stream_complete to asyncio.to_thread(complete)
  so extended thinking works and so only the reply field is shown to
  the parent (not the raw JSON wrapper)

To enable extended thinking set THINKING_BUDGET=8000 in .env.

https://claude.ai/code/session_01SUWzMzFvSfWiHXA2p6rPg9

.env.example

@@ -15,6 +15,12 @@
 #   gemini/gemini-2.0-flash
 AI_MODEL=anthropic/claude-opus-4-6
 
+# Extended thinking — Anthropic models only (leave unset to disable).
+# Enables a reasoning phase before the model's reply, which improves
+# accuracy on age calculations, logic-heavy questions, and edge cases.
+# Recommended value: 8000 (tokens). Must be less than max_tokens.
+# THINKING_BUDGET=8000
+
 # ---------------------------------------------------------------
 # API Keys — set the one matching your chosen model's provider
 # ---------------------------------------------------------------