Fix assignee mechanism.

**Root cause:** The `create_issue` API call failed because `copilot-swe-agent` is not a valid assignee in `github/copilot-sdk-java`. The REST API rejects issue creation entirely when an invalid assignee is specified — so the issue was never created, which also blocked the downstream `assign_to_agent` step.

**Fixes made (3 files):**

1. reference-impl-sync.md — Removed `assignees: [copilot-swe-agent]` from the `create-issue` safe-output config. The `assign_to_agent` safe-output (already configured) is the correct mechanism for assigning Copilot to work on the issue in agentic workflows.

2. reference-impl-sync.lock.yml — Regenerated via `gh aw compile reference-impl-sync`.

3. reference-impl-sync.yml — Removed `--assignee "copilot-swe-agent"` from the `gh issue create` command in the non-agentic fallback workflow.

The issue creation will now succeed without an assignee, and the agentic workflow's `assign_to_agent` tool will handle assigning Copilot to work on it afterward.

.github/workflows/reference-impl-sync.md

@@ -24,7 +24,6 @@ tools:
 safe-outputs:
   create-issue:
     title-prefix: "[reference-impl-sync] "
-    assignees: [copilot-swe-agent]
     labels: [reference-impl-sync]
     expires: 6
   close-issue:

.github/workflows/reference-impl-sync.yml

@@ -129,13 +129,12 @@ jobs:
 
           Follow the [agentic-merge-reference-impl](.github/prompts/agentic-merge-reference-impl.prompt.md) prompt to port these changes to the Java SDK."
 
-          # Create the issue and assign to Copilot coding agent
+          # Create the issue (Copilot coding agent picks it up via the label)
           ISSUE_URL=$(gh issue create \
             --repo "$REPO" \
             --title "Reference implementation sync: ${COMMIT_COUNT} new commits (${DATE})" \
             --body "$BODY" \
-            --label "reference-impl-sync" \
-            --assignee "copilot-swe-agent")
+            --label "reference-impl-sync")
 
           echo "issue_url=$ISSUE_URL" >> "$GITHUB_OUTPUT"
           echo "✅ Issue created and assigned to Copilot coding agent: $ISSUE_URL"