Implement email-address-based conversation matching

Closes the gap where parents sending a new email (instead of replying)
would lose their registration progress. All changes follow the
email-based-conversation-matching OpenSpec change.

Key changes
-----------
storage/json_store.py
  - normalize_email() helper (lowercase + trim)
  - Conversations now keyed by sender email address, not thread ID
  - Versioned registration storage: data/registrations/<email>/v<N>_<ts>.json
  - current.json always reflects the latest version
  - save_registration() returns (email_key, version) tuple
  - save_registration_version() for updates with change_summary
  - get_registration_history() returns all versions in order

models/conversation.py
  - Added last_inbound_message_id field for reply threading (not matching)

channels/email_channel.py
  - fetch_unread_messages() no longer exposes thread_id
  - Conversation matching removed from channel layer (now in agent)
  - Removed _resolve_thread_id() — threading headers kept for SMTP only

agent/core.py
  - process_message() takes parent_email + inbound_message_id (no thread ID)
  - Looks up conversation by normalized email address
  - Post-completion handler: detects intent (question / update / new_child)
  - Registration updates: diffs old vs new, versions storage, notifies admin

agent/prompts.py
  - build_system_prompt() dispatches to registration or post-completion prompt
  - Post-completion prompt guides LLM to return intent field
  - Reminder language updated: no expiration threats

notifications/notifier.py
  - notify_admin() accepts version parameter
  - notify_registration_update() sends "Registration Updated" emails with diff
  - _build_update_body() includes field-level old→new change summary

main.py
  - Poll loop passes parent_email + inbound_message_id to agent (no thread_id)

https://claude.ai/code/session_01HaUFs7SaLD5SoiuGCY27Tw

src/channels/email_channel.py

@@ -2,9 +2,14 @@
 
 Handles:
 - Polling the inbox for unread messages (IMAP)
-- Thread tracking via Message-ID / In-Reply-To / References headers
-- Sending reply emails (SMTP) with proper threading headers
+- Conversation matching by sender email address (NOT by thread headers)
+- Sending reply emails (SMTP) with proper threading headers for email clients
 - Stripping quoted reply text so the agent only sees the new content
+
+Threading headers (Message-ID, In-Reply-To, References) are preserved for
+outbound replies so messages appear threaded in Gmail/Outlook, but they are
+NOT used to identify which conversation an incoming message belongs to.
+Conversation matching is exclusively by normalized sender email address.
 """
 
 import email
@@ -126,13 +131,15 @@ class EmailChannel:
         """Poll the inbox and return all unread messages as structured dicts.
 
         Each dict contains:
-            from          — sender email address
+            from          — sender email address (use this as conversation key)
             subject       — decoded subject line
-            message_id    — Message-ID header of this email
-            in_reply_to   — In-Reply-To header (may be empty)
-            references    — References header (may be empty)
-            thread_id     — canonical ID for the email thread
+            message_id    — Message-ID of this inbound email (for reply threading)
+            in_reply_to   — In-Reply-To header (for reply threading, may be empty)
+            references    — References header (for reply threading, may be empty)
             body          — stripped plain-text body (quoted text removed)
+
+        Note: ``thread_id`` is no longer returned. Conversation matching is done
+        by ``from`` (sender email address), not by threading headers.
         """
         messages: list[dict] = []
         try:
@@ -161,8 +168,6 @@ class EmailChannel:
                         imap.store(num, "+FLAGS", "\\Seen")
                         continue
 
-                    thread_id = self._resolve_thread_id(message_id, in_reply_to, references)
-
                     messages.append(
                         {
                             "from": from_addr,
@@ -170,7 +175,6 @@ class EmailChannel:
                             "message_id": message_id,
                             "in_reply_to": in_reply_to,
                             "references": references,
-                            "thread_id": thread_id,
                             "body": body,
                         }
                     )
@@ -274,16 +278,3 @@ class EmailChannel:
         conn.login(self._username, self._password)
         return conn
 
-    @staticmethod
-    def _resolve_thread_id(message_id: str, in_reply_to: str, references: str) -> str:
-        """Determine the canonical thread ID from email threading headers.
-
-        The root of the thread is the first message ever sent — its ID is the
-        first token in the References header (oldest first convention).
-        """
-        if references:
-            first_ref = references.strip().split()[0]
-            return first_ref
-        if in_reply_to:
-            return in_reply_to.strip()
-        return message_id.strip() or f"<unknown-{time.time():.0f}>"