Pin Agentic Triage to a supported Copilot model (#11826)

Agentic Triage failed when repository or organization defaults selected
a model unavailable to the `agentic-workflows` integration.

- **Workflow configuration**
  - Pin the Copilot engine to `claude-sonnet-5`.
  - Prevent external model defaults from overriding the workflow.

- **Generated workflow**
  - Regenerate `issue-triage.lock.yml` with the pinned model.

<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes #11808

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: timotheeguerin <1031227+timotheeguerin@users.noreply.github.com>

Author: Copilot

.github/workflows/issue-triage.lock.yml

@@ -43,7 +43,9 @@ tools:
 
 timeout-minutes: 10
 source: githubnext/agentics/workflows/issue-triage.md@346204513ecfa08b81566450d7d599556807389f
-engine: copilot
+engine:
+  id: copilot
+  model: claude-sonnet-5
 ---
 
 # Agentic Triage