fix: handle SIGPIPE in Fetch PR changed files step to prevent missing delimiter error (#1651)

When the PR diff exceeds 8000 bytes, `gh api ... | head -c 8000` causes
SIGPIPE (exit 141) on the gh api process. GitHub Actions bash uses
`-eo pipefail`, so the pipeline failure aborts the subshell before
writing the closing heredoc delimiter to GITHUB_OUTPUT, resulting in:

  ##[error]Invalid value. Matching delimiter not found 'GHAW_PR_FILES_...'

Fix: add `|| true` after `head -c 8000` to swallow the SIGPIPE/pipefail
error, ensuring the closing delimiter is always written to GITHUB_OUTPUT.

Fixes failing job: https://github.com/github/gh-aw-firewall/actions/runs/23964770751/job/69902358339

Agent-Logs-Url: https://github.com/github/gh-aw-firewall/sessions/c8b8f958-f52e-4390-af7d-f094e20e68da

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

Author: Copilot

.github/workflows/security-guard.lock.yml

@@ -32,7 +32,7 @@ steps:
         echo "PR_FILES<<${DELIM}"
         gh api "repos/${GH_REPO}/pulls/${PR_NUMBER}/files" \
           --paginate --jq '.[] | "### " + .filename + " (+" + (.additions|tostring) + "/-" + (.deletions|tostring) + ")\n" + (.patch // "") + "\n"' \
-          | head -c 8000
+          | head -c 8000 || true
         echo "${DELIM}"
       } >> "$GITHUB_OUTPUT"
     env: