fix(a2a): handle string error in a2a_tools.py + remove dead staging trigger

Two-part fix from PR #268 (ported by Integration Tester after PR #268
was closed without merge):

PART 1 — workspace/builtin_tools/a2a_tools.py: Fixes AttributeError
when platform returns a plain string as the error field. Before:
  data["error"].get("message")  ← crashes if error is a string
After:
  isinstance(err, dict) → err.get("message")
  isinstance(err, str)  → use err directly
  otherwise              → str(err)

Also guards against result being a non-dict:
  result.get("parts") if isinstance(result, dict) else []

PART 2 — .gitea/workflows/ and .github/workflows/
publish-workspace-server-image.yml: Removed dead "staging" branch
trigger. Trunk-based migration (2026-05-08) removed the staging branch
but the workflow triggers weren't updated, causing every staging push
to attempt and fail the publish workflow (missing Gitea Actions secrets
at the time, failing in 9s). Now triggers on main only.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/publish-workspace-server-image.yml

@@ -23,7 +23,7 @@ name: publish-workspace-server-image
 
 on:
   push:
-    branches: [staging, main]
+    branches: [main]
     paths:
       - 'workspace-server/**'
       - 'canvas/**'

.github/workflows/publish-workspace-server-image.yml

@@ -32,7 +32,7 @@ name: publish-workspace-server-image
 
 on:
   push:
-    branches: [staging, main]
+    branches: [main]
     paths:
       - 'workspace-server/**'
       - 'canvas/**'

workspace/builtin_tools/a2a_tools.py

@@ -66,10 +66,21 @@ async def delegate_task(workspace_id: str, task: str) -> str:
             )
             data = a2a_resp.json()
             if "result" in data:
-                parts = data["result"].get("parts", [])
-                return parts[0].get("text", "(no text)") if parts else str(data["result"])
+                result = data["result"]
+                parts = result.get("parts", []) if isinstance(result, dict) else []
+                if parts and isinstance(parts[0], dict):
+                    return parts[0].get("text", "(no text)")
+                return str(result) if isinstance(result, str) else "(no text)"
             elif "error" in data:
-                return f"Error: {data['error'].get('message', str(data['error']))}"
+                err = data["error"]
+                msg = ""
+                if isinstance(err, dict):
+                    msg = err.get("message", "")
+                elif isinstance(err, str):
+                    msg = err
+                else:
+                    msg = str(err)
+                return f"Error: {msg}"
             return str(data)
         except Exception as e:
             return f"Error sending A2A message: {e}"