fix(scripts): one shell and one environment for every checker (improvise-f9qf)

`scripts/pr-reviews-test.sh` passed 19/19 locally and 2/19 in CI on the same
commit, because a multi-assignment `local` is not left-to-right on bash 5
while macOS's bash 3.2 tolerates it. The obvious half of the fix is pinning
the shell. The half that decides the design is not.

.envrc DOES already put a nix bash 5.3.15 on PATH, which is CI's semantics --
and it is not enough. `pkgs.mkShell` exports `name`, `out`, `outputs`,
`system`, `shell`, `builder`, `stdenv`, `phases` and about twenty more
generically-named variables that CI does not have, so a script reading one by
accident under `set -u` is SILENT in the dev shell and fatal in CI. That is
exactly what happened: the broken clause read `$name`, and in the dev shell
`$name` was `nix-shell-env`. Measured at 051f541^ on one machine:

  macOS /bin/bash 3.2                             19 passed, 0 failed
  direnv exec . ./scripts/pr-reviews-test.sh      19 passed, 0 failed
  the same, with `name` unset                      2 passed, 17 failed

Two reasons for green, one of them the correct interpreter. A dev shell is a
third environment, not a reproduction of CI.

A nix shebang was measured and declined, on availability rather than speed:
warm, `nix shell nixpkgs#bash --command true` is 61.8ms against an ambient
5.1ms, and `nix-shell -p bash --run true` -- the manpage form -- is 675.2ms,
132x, inside git's merge machinery for beads-merge-driver.sh. But only the
`kb` job installs nix; build, test, wasm, js, harness-lint and browser have
none, so a nix shebang needs an installer step on three more jobs to reach a
bash those runners already ship. zsh is not a shebang change and the runner
does not have it either. Both declined in context/code/tooling.md section 5.

What lands instead pins interpreter and environment together, in one place:
scripts/checkers.sh is the one entry point for every check_the_checker, used
verbatim by CI and by the local gate. It pins a bash >= 5 (ambient first, nix
only when the ambient one is older, exit 3 when neither), scrubs the
environment to an allowlist, and refuses any shell shebang in scripts/ that
is not `#!/usr/bin/env bash`. The pinned bash goes on the front of PATH so
inner invocations get it too -- which is where the fault actually lived.

The second undetected split is closed with it: the four `#!/bin/sh` and one
`#!/usr/bin/env sh` scripts are now bash, and the six places that ran a
script through an explicit `sh` -- overriding the callee's own shebang, which
is the same disease -- now invoke it directly.

Verification, injected against the real script: restoring pr-reviews'
multi-assignment `local` leaves `sh scripts/pr-reviews-test.sh` at 22/22 and
`direnv exec . ./scripts/pr-reviews-test.sh` at 22/22, while
`./scripts/checkers.sh pr-reviews` reports 2 passed, 20 failed, exit 1 --
matching the 2/19 CI actually reported. scripts/checkers-test.sh is twelve
cases, each injecting rather than asserting; the pair that carries it runs
one fixture both ways, so the `set -u` case is a test of the runner and not
of `set -u`.

The old local gate was `for t in scripts/*-test.sh; do sh "$t"; done`, which
overrides every shebang with /bin/sh -- so it ran every suite under bash 3.2
in sh-mode regardless of what the file said. It is replaced by
`./scripts/checkers.sh`.

Workspace suite unchanged at 1689 passing over 38 binaries.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH

web/assemble.sh

@@ -84,7 +84,7 @@ sedi "s|'improvise_worker_server_bg\.wasm'|'improvise_worker_server_bg.wasm?v=${
 # script so its refusals can be exercised without fabricating a web/pkg
 # (improvise-w8ih). Its argument is FILE PATTERN, five times -- the page's own
 # #build-version among them (improvise-oujb).
-sh "$root/scripts/assemble-stamp.sh" \
+"$root/scripts/assemble-stamp.sh" \
   "$app/index.html" "id=\"build-version\">${token}<" \
   "$app/app.js" "pkg/improvise_wasm_client.js?v=${token}" \
   "$app/worker.js" "worker-pkg/improvise_worker_server.js?v=${token}" \