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>

config.json

@@ -26,10 +26,10 @@
   },
   "heartbeat": {
     "enabled": true,
-    "interval_minutes": 30,
+    "interval_minutes": 120,
     "channel": "echo-core",
     "model": "haiku",
-    "quiet_hours": [23, 8],
+    "quiet_hours": [23, 7],
     "checks": {
       "email": true,
       "calendar": true,