Security: Add .gitignore, remove sensitive files from tracking

- Added comprehensive .gitignore for secrets, tokens, session data
- Untracked openclaw.json (contains API tokens)
- Untracked device-auth.json, paired.json
- Untracked session .jsonl files (conversation history)
- Untracked cron run logs

Note: Files still exist locally, just no longer version controlled

identity/device-auth.json

@@ -1,16 +0,0 @@
-{
-  "version": 1,
-  "deviceId": "a964ef10e2ffd5bc0516040d9374165f44d5503ac06dbf1725e7b794989b25bd",
-  "tokens": {
-    "operator": {
-      "token": "37918a43e1044131a00ffd093c12127d",
-      "role": "operator",
-      "scopes": [
-        "operator.admin",
-        "operator.approvals",
-        "operator.pairing"
-      ],
-      "updatedAtMs": 1770304320374
-    }
-  }
-}