fix(pr-sous-chef): use sonnet alias for pr-processor sub-agent to resolve HTTP 400 (#42634)

Author: Copilot

.github/workflows/pr-sous-chef.lock.yml

@@ -369,7 +369,7 @@ safeoutputs noop --message "processed=4; skipped_checks_running=0; skipped_last_
 ## agent: `pr-processor`
 ---
 description: Processes one PR with minimal API calls and returns skip/nudge decisions
-model: claude-sonnet-4.5
+model: sonnet
 ---
 Given one PR number and compact metadata:
 

.github/workflows/pr-sous-chef.md

@@ -29,7 +29,7 @@ func TestPRSousChefWorkflowAddCommentTargetContract(t *testing.T) {
 	assert.Contains(t, text, "include an explicit unresolved-reviews list", "Workflow should require explicit unresolved review listing in nudge comments")
 	assert.Contains(t, text, "Process at most **5 PRs** per run.", "Workflow should cap per-run PR processing to 5")
 	assert.Contains(t, text, "Make at most 8 tool calls total.", "Sub-agent should have a hard tool-call budget")
-	assert.Contains(t, text, "model: claude-sonnet-4.5", "Sub-agent should use a Sonnet model")
+	assert.Contains(t, text, "model: sonnet", "Sub-agent should use a Sonnet model alias")
 	assert.Contains(t, text, "skip_reason: \"sub_agent_error\"", "Workflow should skip failed sub-agent responses without retry")
 	assert.Contains(t, text, "eligible_count=", "fetch-prs step must export eligible_count output")
 	assert.Contains(t, text, ".prs | length", "eligible_count should reflect the number of eligible PRs")