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 + luacheck --max-line-length 130 green + regression test.

[Copilot]

Pull request overview

This PR introduces treefmt-based formatting to the repo via Nix (including nix fmt, a formatting check exposed as a flake check, a pre-commit hook installed in the dev shell, and a CI “format check” step), and applies an initial formatting pass across PureScript, Lua FFI, and Dhall sources.

Changes:

• Add treefmt-nix integration to the flake (formatter, checks.formatting) and a devShell pre-commit hook that runs nix fmt.
• Add formatter configuration files (treefmt.nix, .tidyrc.json, .lua-format) and update CI to enforce formatting.
• Reformat PureScript/Lua/Dhall sources to match the new formatting setup.

Reviewed changes

Copilot reviewed 19 out of 24 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
treefmt.nix	Adds treefmt configuration for Nix/Dhall/PureScript/Lua formatting.
flake.nix	Wires treefmt-nix into the flake and installs a devShell pre-commit hook.
flake.lock	Adds the treefmt-nix input lock entry.
.github/workflows/ci.yml	Adds a formatting check step; updates luacheck line length.
.gitignore	Ensures formatter config dotfiles are not ignored.
.tidyrc.json	Adds purs-tidy configuration.
.lua-format	Adds LuaFormatter configuration.
AGENTS.md	Documents formatting/linting commands and the formatting approach.
spago-test.dhall	Reformats Dhall configuration.
bench/Main.purs	Reformats a long string literal for purs-tidy output.
src/Data/String/Common.lua	Reformats Lua FFI code.
src/Data/String/CodePoints.lua	Reformats Lua FFI code.
src/Data/String/CodeUnits.purs	PureScript formatting pass (layout/indentation).
src/Data/String/CodePoints.purs	PureScript formatting pass (layout/indentation).
src/Data/String/NonEmpty/Internal.purs	PureScript formatting pass (layout/indentation).
src/Data/String/NonEmpty/CodeUnits.purs	PureScript formatting pass (layout/indentation).
src/Data/String/NonEmpty/CodePoints.purs	PureScript formatting pass (layout/indentation).
src/Data/String/NonEmpty/CaseInsensitive.purs	PureScript formatting pass (layout/indentation).
src/Data/Char/Gen.purs	PureScript formatting pass (layout/indentation).
test/Test/Data/String.purs	PureScript formatting pass (layout/indentation).
test/Test/Data/String/CodeUnits.purs	PureScript formatting pass (layout/indentation).
test/Test/Data/String/CaseInsensitive.purs	PureScript formatting pass (layout/indentation).
test/Test/Data/String/NonEmpty.purs	PureScript formatting pass (layout/indentation).
test/Test/Data/String/NonEmpty/CodeUnits.purs	PureScript formatting pass (layout/indentation).

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