feat(heartbeat): save emails to KB + fix memory symlink access

- heartbeat saves unread whitelisted emails via email_process --save --json
- fix: add --add-dir so Claude CLI subprocess can access memory/ symlink
- email_check/process: use BODY.PEEK[] to avoid marking emails as read
- email_process: simplify credential loading via credential_store only
- config: heartbeat interval 30→120min, quiet hours end 08→07

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

tools/email_check.py

@@ -77,7 +77,7 @@ def check_inbox(unread_only=True, limit=10):
         
         emails = []
         for eid in reversed(email_ids):  # Newest first
-            status, msg_data = mail.fetch(eid, "(RFC822)")
+            status, msg_data = mail.fetch(eid, "(BODY.PEEK[])")
             if status != "OK":
                 continue