CLI: a closed stdout pipe exits 141 silently (was E_INTERNAL); example avoids head under pipefail

Found by the P10 fresh-VM run: `simurg inspect … | head` raised BrokenPipeError, reported as
an internal error, and stopped examples/gsa_demo.sh (set -o pipefail). run_guarded now flushes
stdout inside the guard and maps BrokenPipeError to ExitCode.SIGPIPE (141) without a message.
p10.sh: SKIP_DOCKER/SKIP_CONDA, clearer digest report; p10t.sh runs build+pip+tutorial only.

src/simurg/core/errors.py

@@ -23,6 +23,7 @@ class ExitCode(IntEnum):
     PARTIAL = 3  # >= 1 sample failed and >= 1 succeeded
     ALL_FAILED = 4  # run completed but no sample succeeded
     SIGINT = 130
+    SIGPIPE = 141  # the reader of stdout went away (e.g. `simurg inspect … | head`); nothing is reported
     SIGTERM = 143