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.

[Copilot]

Pull request overview

This PR rolls out repository-wide formatting support using treefmt-nix, wiring it into the flake so nix fmt works locally, CI enforces formatting, and the dev shell installs a pre-commit hook to prevent unformatted commits.

Changes:

• Add treefmt-nix integration to flake.nix (formatter output + checks.formatting) and a dev-shell pre-commit hook.
• Add formatter configs (treefmt.nix, .tidyrc.json, .lua-format) and document usage in AGENTS.md.
• Update CI to run a format check and apply the initial formatter pass to PureScript/Lua sources.

Reviewed changes

Copilot reviewed 8 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
treefmt.nix	Defines treefmt programs and custom formatters (purs-tidy, LuaFormatter) plus global excludes.
flake.nix	Adds treefmt-nix input, exposes formatter/checks.formatting, installs a pre-commit hook in shellHook.
.github/workflows/ci.yml	Adds nix fmt && git diff --exit-code format check step; updates luacheck line length.
AGENTS.md	Documents new format command and updated luacheck settings.
.tidyrc.json	Adds purs-tidy configuration.
.lua-format	Adds LuaFormatter configuration for FFI sources.
.gitignore	Ensures new dotfile configs are tracked.
src/Test/Assert.purs	Pure formatting changes from purs-tidy.
src/Test/Assert.lua	Pure formatting changes from LuaFormatter.
flake.lock	Locks the new treefmt-nix input.

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