fix: remove the issue/PR triage and fix agent workflows

Remove the issue-analyze, issue-fix, and pr-analyze GitHub Actions
workflows. They ran an automated agent over untrusted issue and PR
content with broad repository credentials; deleting them removes that
exposure while a safer design is considered.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 929246616

Author: GWeale