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

main.py

@@ -63,7 +63,13 @@ def build_components(config: Config):
         cc_emails=[e.strip() for e in config.admin_email_cc.split(",") if e.strip()],
     )
 
-    agent = EmailAgent(model=config.ai_model, kb=kb, store=store, notifier=notifier)
+    agent = EmailAgent(
+        model=config.ai_model,
+        kb=kb,
+        store=store,
+        notifier=notifier,
+        thinking_budget=config.thinking_budget,
+    )
 
     channel = EmailChannel(
         imap_host=config.imap_host,