Add prelude.pl: a Prolog file consulted at conversation start

[vbergeron]

Point MANIFOLD_PRELUDE (config :manifold, :prelude_path) at a .pl file and
it is consult/1'ed into every conversation's own engine at boot, ahead of
replay and ahead of the unknown=fail flag — so a directive that references
something undefined still throws instead of silently failing, and anything
the prelude defines is already there for the first replayed clause or live
turn to see.

It is background knowledge shared by every conversation's engine, not a
conversation input: never part of kb_snapshot, never written to the
per-conversation log, and reconsulted fresh into each conversation's own
swipl process (never shared, per the per-conversation-engine architecture).
A prelude that fails to load — missing file, an erroring directive — fails
that conversation's startup the same way a dead swipl does, surfacing as
error{prolog_unavailable} rather than booting a conversation silently short
of the rules it was told to have.

Unset by default, so no existing deployment or test is affected.