Appends the R3 section to agents/steelbeak/work/queue-227/BUILD.md: the
R2 red on dummy links, the fixes, how the TMPDIR limit is derived, the
namespace containment and control table, runs, signal probes, the tip
check on 5df0e9d6f and follow-ups. One line each in BUILD-LOG.md and
docs/SESSIONS.md. Earlier entries are unchanged.