setenv: preserve OMNeT++'s bin/ and python/ after venv activation

[tabgab]

Summary

opp_repl's setenv activates the project's .venv near the top:

if [ -f "$OPP_REPL_ROOT/.venv/bin/activate" ]; then
    . "$OPP_REPL_ROOT/.venv/bin/activate"
fi

bin/activate restores a snapshot of $PATH taken when the venv was
created. For most users that snapshot does NOT include OMNeT++'s
bin/, so after:

source ~/omnetpp/setenv -q         # puts OMNeT++ bin on PATH
source ~/opp_repl/setenv           # venv activate erases it

opp_makemake, opp_run, opp_scavetool and every other OMNeT++
binary falls off PATH, even though the user sourced things in the
"natural" order (dependency first, then dependent). Typical symptom:

$ opp_makemake --help
bash: opp_makemake: command not found

The workaround is to source opp_repl/setenv first — counter-intuitive
since opp_repl depends on OMNeT++, not the other way around.

Change

Capture $__omnetpp_root_dir/{bin,python} BEFORE the venv activation,
then re-prepend them to $PATH / $PYTHONPATH AFTER activation (only
if not already present). Result: both sourcing orders work.

Verified

$ env -i HOME=$HOME PATH=/usr/bin:/bin bash
$ source ~/omnetpp/setenv -q
$ source ~/omnetpp/misc/opp_repl/setenv    # patched
$ command -v opp_makemake opp_run opp_repl
/home/gabor/omnetpp-aipre/bin/opp_makemake
/home/gabor/omnetpp-aipre/bin/opp_run
/home/gabor/omnetpp-aipre/misc/opp_repl/.venv/bin/opp_repl

All three resolve, and opp_repl's own bin/ is still prepended on top
(so opp_repl, opp_run_simulations etc. take precedence for opp_repl-owned
tools).

Context

Found while verifying an Agent Skills pack
(tabgab/opp_repl-skill) that
walks agents through testprompt.txt-style end-to-end scenarios. This
PATH bug was one of the three remaining friction points weak models
couldn't navigate on their own.