Delete the Janet host — Chez is the sole substrate

Remove the Janet seed (src/jolt/*.janet: reader, value layer, vars/ns, the
tree-walking interpreter, the Janet backend, the optimizing compiler), the
Janet->Scheme cross-compiler (host/chez/{driver,emit,jolt-chez}.janet),
bin/jolt-chez, the jpm build (project.janet) and the Janet test runner
(run-tests.janet), plus the entire Janet test suite. jolt now builds and runs
on Chez alone: bin/joltc off the checked-in seed, bootstrap.ss to rebuild it.

The portable Clojure stays: jolt-core/**, host/chez/**.ss, and the stdlib +
tooling under src/jolt/clojure + src/jolt/jolt (read by the seed build, no
Janet). The gate is 'make test' (self-host, corpus, unit, cli smoke, certify).
Drop the sci and clojure-test-suite submodules (used only by deleted Janet
integration tests); irregex stays.

Filesystem corpus/unit cases that probed project.janet now probe README.md.

jolt-cf1q.6

bin/jolt-chez

@@ -1,7 +0,0 @@
-#!/bin/sh
-# -e-capable jolt-chez launcher (jolt-9ziu). Runs from the repo root so the
-# assembled prelude can load host/chez/rt.ss by relative path.
-#   JOLT_BIN=bin/jolt-chez janet test/chez/run-corpus.janet
-root="$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)"
-cd "$root" || exit 1
-exec janet host/chez/jolt-chez.janet "$@"