fix: skip add_labels in build-test on workflow_dispatch (#1914)

The build-test workflow fails on workflow_dispatch because the agent
calls add_labels when there's no PR context. The safe_outputs handler
for add_labels errors with 'No issue/PR number available' unlike
add_comment which gracefully skips.

Update the prompt to instruct the agent to only add labels when
triggered by a pull_request event.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lpcox

.github/workflows/build-test.md

@@ -242,7 +242,7 @@ After completing ALL tasks, add a **single comment** to the current pull request
 
 **Overall: X/8 ecosystems passed — PASS/FAIL**
 
-If ALL tests across all ecosystems pass, add the label `build-test` to the pull request.
+If ALL tests across all ecosystems pass **and** this run was triggered by a pull request (not `workflow_dispatch`), add the label `build-test` to the pull request.
 If ANY test fails, report the failure with error details below the table.
 
 ## Error Handling