Migrate purescript-lua-st off the JavaScript-era tooling onto the
package-set canon: an overlay flake (purs 0.15.16, spago 0.21.0, Lua 5.1,
pslua at the current main), hardened CI, scripts/build, AGENTS.md and
CLAUDE.md, a clean .gitignore, and the org package-set URL. Drop the
.eslintrc, package.json and the JavaScript FFI files.

Add a systematic test suite: spago-test.dhall compiles test/Main.purs to
dist/test.lua through pslua and scripts/test runs it under Lua 5.1. It
covers STRef new/read/write/modify/modify', ST.run recursion, ST.for
(hi-exclusive, lo-inclusive, empty range), ST.while and ST.foreach.

Fixes surfaced by the new tests and luacheck: ST.for looped over an
inclusive upper bound (for i = lo, hi) though the contract is
hi-exclusive, so it ran one extra iteration -- corrected to hi - 1.
Declare the effect dependency that Control.Monad.ST.Global needs, and
wrap an over-long line in the Uncurried FFI so luacheck passes.

feat: bring st to the canon and add ST unit tests

Wire treefmt via treefmt-nix: nixfmt, dhall format, purs-tidy (.tidyrc.json)
and LuaFormatter for the FFI (.lua-format, kept over StyLua because it
preserves the parentheses pslua's parser needs). `nix fmt` formats; the dev
shell installs a content-based pre-commit hook and CI runs
`nix fmt && git diff --exit-code` (content-based, since the in-place
formatters bump mtime and would trip treefmt --fail-on-change). Lua lines
budget 130 cols, matching the raised `luacheck --max-line-length`. The bulk
of the diff is the first format pass.

chore: add treefmt formatting (nix fmt + pre-commit + CI check)

Replace the dev-shell installer that wrote .git/hooks/pre-commit with a
tracked .githooks/pre-commit wired via `git config core.hooksPath .githooks`.

Fixes three issues with the old hook: it gated on `[ -d .git ]` (false in
worktrees/submodules where .git is a file), it clobbered any existing
.git/hooks/pre-commit on every `nix develop`, and `nix fmt … || exit 0`
swallowed real formatter failures. The new hook skips only when `nix` is
absent (CI is the authoritative gate) and otherwise blocks on a `nix fmt`
failure or reformat.

Toolchain consistency: pin the pslua dev flake input to the current
compiler main across the ecosystem. flake.lock only; the package set
consumes sources + FFI, not this dev input, so no re-tag is needed.