Add iMessage threading, typing indicators, and fix tool-use response extraction

- Native reply threading: inbound replies continue their conversation via
  SendBlue message_handle lookup; standalone messages create a new conversation
- Our responses are sent with reply_to so they thread visually in iMessage
- Typing indicator fires before agent invocation and stops in a finally block
- Add extract_agent_text() to handle list-type AIMessage content and strip
  <think> tokens from Qwen3 — applied to iMessage, WhatsApp, and web chat
- ConversationMessage gains a nullable message_handle field (migration needed)
- /threads command lists recent iMessage conversations

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

utils/strip_markdown.py

@@ -1,5 +1,26 @@
 import re
 
+_THINK_RE = re.compile(r"<think>.*?</think>", re.DOTALL)
+
+
+def extract_agent_text(content) -> str:
+    """Extract plain text from a LangChain AIMessage content.
+
+    Handles both str and list-of-blocks content (the latter occurs with some
+    tool-using or vision models), and strips <think>...</think> tokens emitted
+    by reasoning models like Qwen3.
+    """
+    if isinstance(content, list):
+        text = "".join(
+            block.get("text", "")
+            if isinstance(block, dict) and block.get("type") != "thinking"
+            else (str(block) if not isinstance(block, dict) else "")
+            for block in content
+        )
+    else:
+        text = str(content) if content is not None else ""
+    return _THINK_RE.sub("", text).strip()
+
 
 def strip_markdown(text: str) -> str:
     """Strip markdown formatting from text for plain-text channels like iMessage."""