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 result.get("parts") against non-dict result.
Includes fix for issue #279: empty-parts regression where
{"parts": []} returned "(no text)" instead of str(result).

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 were not updated.

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 +66,25 @@ 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)")
+                # Empty parts list (e.g. {"parts": []}) should return str(result),
+                # not "(no text)" — preserves pre-fix behavior (#279 regression fix).
+                if isinstance(result, dict) and result.get("parts") == []:
+                    return str(result)
+                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}"