chore: prepare desktop 0.15.4 and agent 0.15.1 releases

Document layered Runtime and model selection, separate system-task usage, and Continue history handling. Record schema 49 backup requirements and current-source Host validation with explicit failed-provider attempts.

Release note: Desktop 0.15.4 supports layered Runtime/model choices and separate system-task usage; update Desktop before Agent 0.15.1 to receive the remote Continue history fix.

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "goodbuddy",
-  "version": "0.15.3",
+  "version": "0.15.4",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "goodbuddy",
-      "version": "0.15.3",
+      "version": "0.15.4",
       "license": "0BSD",
       "dependencies": {
         "@agentclientprotocol/sdk": "0.25.1",