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

[Unisay]

Roll the ecosystem formatting setup (ADR 0007, piloted in purescript-lua/purescript-lua-effect#6) onto this fork.

Tooling. Copies treefmt.nix/.tidyrc.json/.lua-format and wires treefmt-nix into the flake: nix fmt formats, checks.formatting is exposed, the dev shell installs a content-based pre-commit hook, and CI gains a format-check step. The check is content-based (nix fmt && git diff --exit-code) rather than treefmt --ci, since the in-place formatters bump mtime even when content is unchanged. LuaFormatter is kept over StyLua because it preserves the parentheses pslua's foreign-file parser requires; luacheck --max-line-length raised to 130 to match.

Diff. The bulk is the first nix fmt pass over *.purs and the *.lua FFI. Verified locally: build + regression test + luacheck --max-line-length 130 green.

[Copilot]

Pull request overview

This PR introduces repo-wide, Nix-flake-integrated formatting via treefmt (exposed as nix fmt), along with a CI format check and a dev-shell-installed pre-commit hook, and applies the first formatting pass across PureScript and Lua FFI sources.

Changes:

• Add treefmt-nix integration in flake.nix (formatter + checks.formatting) and add a CI “Format check” step.
• Add treefmt configuration (treefmt.nix) plus formatter configs for purs-tidy and LuaFormatter (.tidyrc.json, .lua-format), and update docs/ignore rules accordingly.
• Apply formatting updates across existing *.purs sources, Lua FFI, regression tests, and benchmarks; align luacheck line-length budget to 130.

Reviewed changes

Copilot reviewed 17 out of 22 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
treefmt.nix	Defines treefmt programs for Nix/Dhall/PureScript/Lua and global excludes.
flake.nix	Wires treefmt-nix into the flake, exposes formatter/check, and installs a pre-commit hook in the dev shell.
flake.lock	Adds the treefmt-nix flake input lock entry.
.github/workflows/ci.yml	Adds --max-line-length 130 to luacheck and introduces a format-check step.
AGENTS.md	Documents formatting commands and rationale (treefmt + LuaFormatter parentheses requirement).
.tidyrc.json	Adds purs-tidy configuration used by treefmt.
.lua-format	Adds LuaFormatter configuration for Lua FFI formatting constraints.
.gitignore	Ensures new formatter config dotfiles are not ignored.
src/Data/Array.purs	Formatting-only changes (line wraps/indentation).
src/Data/Array.lua	Formatting-only changes to Lua FFI code style.
src/Data/Array/NonEmpty.purs	Formatting-only changes (exports, type signatures, layout).
src/Data/Array/NonEmpty/Internal.purs	Formatting-only changes to derived instance declarations.
src/Data/Array/ST.purs	Formatting-only changes to foreign import type formatting.
src/Data/Array/ST/Iterator.purs	Formatting-only changes to type signature layout.
bench/Data/Array.purs	Formatting-only changes to let layout.
test/regression/array_st.lua	Formatting-only changes to Lua regression test formatting.
test/Test/Data/UndefinedOr.purs	Formatting-only changes to foreign import type layout.
test/Test/Data/Array/ST/Partial.purs	Formatting-only changes (array literal spacing).
test/Test/Data/Array/ST.purs	Formatting-only changes (wrapping/indentation).
test/Test/Data/Array/Partial.purs	Formatting-only changes (array literal spacing).
test/Test/Data/Array/NonEmpty.purs	Formatting-only changes (wrapping/indentation).
test/Test/Data/Array.purs	Formatting-only changes (wrapping/indentation).

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