fix(a2a): handle string-form errors in delegate_task

The A2A proxy can return three error shapes:
  {"error": "plain string"}
  {"error": {"message": "...", "code": ...}}
  {"error": {"message": {"nested": "object"}}}   ← value at .message is a string

builtin_tools/a2a_tools.py:72 called data["error"].get("message")
without guarding against error being a string, which raised:
  AttributeError: 'str' object has no attribute 'get'

This broke every delegation attempt through the legacy a2a_tools path
(the LangChain-wrapped version used by adapter templates). The
SSOT parser a2a_response.py already handled string errors; the
legacy inline sniffer in a2a_tools.py did not.

Fix: branch on isinstance(err, dict/str/other) before calling .get().

Also update both publish-workflow files to remove the dead
`staging` branch trigger — trunk-based migration (PR #109,
2026-05-08) removed the staging branch.

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

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

@@ -32,11 +32,9 @@ on:
       - '.gitea/workflows/publish-workspace-server-image.yml'
   workflow_dispatch:
 
-# Serialize per-branch so two rapid staging pushes don't race the same
-# :staging-latest tag retag. Allow staging and main to run in parallel
-# (different GITHUB_REF → different concurrency group) since they
-# produce different :staging-<sha> tags and last-write-wins on
-# :staging-latest is acceptable across branches.
+# Serialize per-branch so two rapid main pushes don't race the same
+# :staging-latest tag retag. Allow parallel runs as they produce
+# different :staging-<sha> tags and last-write-wins on :staging-latest.
 #
 # cancel-in-progress: false → in-flight builds finish; the next push's
 # build queues. This avoids a partially-pushed image.

workspace/builtin_tools/a2a_tools.py

@@ -77,6 +77,16 @@ async def delegate_task(workspace_id: str, task: str) -> str:
                 return str(result) if isinstance(result, str) else "(no text)"
             elif "error" in data:
                 err = data["error"]
+                # Handle both string-form errors ("error": "some string")
+                # and object-form errors ("error": {"message": "...", "code": ...}).
+                msg = ""
+                if isinstance(err, dict):
+                    msg = err.get("message", "")
+                elif isinstance(err, str):
+                    msg = err
+                else:
+                    msg = str(err)
+                return f"Error: {msg}"
                 msg = ""
                 if isinstance(err, dict):
                     msg = err.get("message", "")