Implement "attach" command and graceful exiting

The target is to implement (mecha)babs support for monitoring entire subject processing job script, so we could do smth like (expressing)

```patch
diff --git a/code/participant_job.sh b/code/participant_job.sh
index a49142d..c071ce2 100755
--- a/code/participant_job.sh
+++ b/code/participant_job.sh
@@ -6,6 +6,9 @@

#SBATCH --time=24:00:00

+DUCT_PREFIX="logs/duct_${subid}_" con-duct attach &
+duct_pid=$!
+
# shellcheck disable=SC1091
source /dartfs/rc/lab/D/DBIC/DBIC/CON/asmacdo/mechababs/.venv/bin/activate

@@ -109,7 +112,6 @@ if [ ! -L "${CONTAINER_JOB}" ]; then
exit 1
fi

-export DUCT_PREFIX="logs/duct_${subid}_"

# Step 1: Run BIDS app via containers-run
datalad containers-run \
@@ -156,4 +158,7 @@ echo '# Push the branch with provenance records:'
# shellcheck disable=SC2154
flock "${DSLOCKFILE}" git push outputstore "${BRANCH}"

-echo SUCCESS
\ No newline at end of file
+
+kill "$duct_pid" # inform to gracefully exit
+
+echo SUCCESS

```

docstring for `attach` would tell user to use `kill` potentially with specific signal (9 or INT) as to e.g. trigger killing without graceful shutdown which should happen on default `kill` which is TERM (15).

and `attach` will just monitor current session (like we have `run --session=current`) and not require a command to run

Start with the code/participant_job.sh example and compare the requested attach behavior with the existing run --session=current mode. Trace the attach command entry point and determine how it monitors the current session without a command. Done means the session can be stopped gracefully with the default signal, while the docstring explains how stronger signals affect shutdown.

Written by the indexing model from the issue text.