sl0ppty: a --script pane is told no session rather than somebody else's

`server.c` set `SL0PPTY_SESSION` for its panes and carried a comment saying the
value was "deliberately unset under `--script`". Nothing unset it. A value never
set is a value inherited, so a script driver started from inside a pane handed its
own panes the *outer* session's name -- and the recipe the skill documents,
`-s ${SL0PPTY_SESSION:-main} cmd`, then sent its commands to a session it was not
in. Not a fallback to a guess: a confident wrong answer, because the name it read
was a real session that really answers.

Both variables now go through one `session_env()`, called by the server with its
name and by the headless driver with NULL, because the two modes have to agree
about what a pane is told and a comment in one of them is not agreement.
`SL0PPTY_BIN` is set there too rather than inherited, so it names the binary that
actually made the pane instead of one that made some pane somewhere -- which is
the whole reason that variable exists.

Found while explaining a stale pane title, from a check in `test_skill.py` that
failed under `make test` and passed on its own. It was not flaky: the suite runs
from inside a pane, where these are set, and the tool I ran it from by hand had an
environment where they were not. A test whose answer depends on where you run it
looks exactly like a race.

So it now passes the lie in deliberately: the session under test is started with
`SL0PPTY_SESSION=somebody-elses-session`, which makes the difference between
"never set" and "cleared" visible from any shell. The mirror check is next to it --
a real session tells its panes its *own* name, even when started from inside
another one -- so the obvious wrong fix, unsetting it in both modes, fails. Both
were verified by making both wrong versions and watching each test bite.

The two title checks now wait for the line instead of sampling after 120ms. That
was not the bug (a shell prints in about 12ms here, even under disk churn) and it
is one assumption fewer.

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