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

src/llm.py

@@ -5,7 +5,12 @@ from collections.abc import Generator
 import litellm
 
 
-async def acomplete(model: str, system: str, messages: list) -> str:
+async def acomplete(
+    model: str,
+    system: str,
+    messages: list,
+    thinking_budget: int | None = None,
+) -> str:
     """Call any LLM asynchronously and return the response text.
 
     This is the async equivalent of ``complete()`` — use this from async
@@ -16,17 +21,30 @@ async def acomplete(model: str, system: str, messages: list) -> str:
         model: litellm model string, e.g. "anthropic/claude-opus-4-6".
         system: System prompt text.
         messages: List of objects with .role and .content attributes.
+        thinking_budget: When set, enables extended thinking (Anthropic models
+            only). See ``complete()`` for details.
 
     Returns:
         The model's reply as a plain string.
     """
     api_messages = [{"role": "system", "content": system}]
     api_messages += [{"role": m.role, "content": m.content} for m in messages]
-    response = await litellm.acompletion(model=model, messages=api_messages, max_tokens=2048)
+
+    kwargs: dict = {"model": model, "messages": api_messages, "max_tokens": 2048}
+    if thinking_budget is not None:
+        kwargs["thinking"] = {"type": "enabled", "budget_tokens": thinking_budget}
+        kwargs["max_tokens"] = thinking_budget + 4096
+
+    response = await litellm.acompletion(**kwargs)
     return response.choices[0].message.content
 
 
-def complete(model: str, system: str, messages: list) -> str:
+def complete(
+    model: str,
+    system: str,
+    messages: list,
+    thinking_budget: int | None = None,
+) -> str:
     """Call any LLM and return the response text.
 
     Args:
@@ -35,13 +53,24 @@ def complete(model: str, system: str, messages: list) -> str:
                environment variable (ANTHROPIC_API_KEY, OPENAI_API_KEY, …).
         system: System prompt text.
         messages: List of objects with .role and .content attributes.
+        thinking_budget: When set, enables extended thinking (Anthropic models
+            only). The value is the token budget for the thinking phase; the
+            final ``max_tokens`` is set to ``thinking_budget + 4096`` so the
+            model has enough room to both think and reply.
 
     Returns:
         The model's reply as a plain string.
     """
     api_messages = [{"role": "system", "content": system}]
     api_messages += [{"role": m.role, "content": m.content} for m in messages]
-    response = litellm.completion(model=model, messages=api_messages, max_tokens=2048)
+
+    kwargs: dict = {"model": model, "messages": api_messages, "max_tokens": 2048}
+    if thinking_budget is not None:
+        kwargs["thinking"] = {"type": "enabled", "budget_tokens": thinking_budget}
+        # max_tokens must exceed budget_tokens or the API returns an error.
+        kwargs["max_tokens"] = thinking_budget + 4096
+
+    response = litellm.completion(**kwargs)
     return response.choices[0].message.content