Migrate the dev shell from easy-purescript-nix to purescript-overlay so plain `nix develop` evaluates (easy-ps pulled EOL nodejs marked insecure, breaking CI). Drop purs-tidy, use the overlay purs 0.15.16 + legacy spago 0.21, pin Lua 5.1 (the pslua target runtime), and replace the inherited bower CI with the canonical nix workflow. (purescript-lua/purescript-lua#54)

chore: align build tooling (purescript-overlay, nix CI, Lua 5.1)

CI: drop accept-flake-config (supply-chain risk; caches already pinned),
run luacheck with --std lua51 --no-unused-args (matches the Lua 5.1 target
and the curried-FFI idiom), invoke the test step via bash so it no longer
needs the execute bit.

AGENTS.md is the single instruction file the coding agents read natively;
CLAUDE.md is a one-line @AGENTS.md import so Claude Code shares it.
Tooling and docs only, no src/ change.

chore: align CI to the hardened canon, add AGENTS.md

Repos moved from Unisay/* to the purescript-lua org. Update the pslua
flake input and lockfile owner (same rev 94c13ce), the packages.dhall
upstream-lua release URL, and the AGENTS.md links, so nothing relies on
the old-owner redirect.

chore: point pslua + package-set links at purescript-lua org

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.

Build against the published purescript-lua package set via
workspace.packageSet.url instead of the old spago.dhall + packages.dhall
(upstream-ps // upstream-lua) merge. The new spago drives spago.yaml/spago.lock
and dropped Dhall.

- spago.dhall/packages.dhall -> spago.yaml (package name lua-control; the new
  spago rejects the purescript- prefix). backend.cmd "true" keeps codegen on
  corefn; the pslua invocation that used to live in spago.dhall's backend now
  runs from scripts/build over the emitted corefn.
- flake.nix: spago 0.21.0 -> 1.x; drop the now-unused dhall dev-shell tool.
- treefmt.nix: drop the dead Dhall formatter. AGENTS.md: spago 1.x, src/packages.json.
- CI action versions bumped to current majors (checkout v5, install-nix v31).
- Remove the upstream PureScript test sources under test/Test (never built: old
  sources globbed src/ only, and they carry JS FFI that doesn't target Lua). The
  Lua regression tests under test/regression stay.

Adds a CHANGELOG.md managed with scriv (fragments in changelog.d/), provides
scriv in the flake dev shell, and records the fork's Lua 5.1 FFI release line.
Where the upstream PureScript changelog existed it is preserved below the fork
section. The AGENTS.md Releasing note now points at the scriv flow instead of
"no changelog entry".

Part of the ecosystem-wide changelog adoption
(purescript-lua/purescript-lua#101); canon in package-set ADR 0009.