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.

Fork specifics. Replaces the fork's bespoke .lua-format with the ecosystem canon (compact one-liners); the nginx FFI keeps its luacheck --globals ngx flag, with --max-line-length raised to 130. dhall format also normalised packages.dhall. Verified locally: build + luacheck --globals ngx --max-line-length 130 green.

[Copilot]

Pull request overview

This PR rolls out standardized formatting infrastructure (treefmt-nix + nix fmt), including a pre-commit hook and a CI formatting check, aligning this fork with the upstream ecosystem formatting setup.

Changes:

• Adds treefmt-nix integration to the flake (formatter output + checks.formatting) and documents formatting usage.
• Introduces formatting configs for PureScript (.tidyrc.json) and Lua FFI (.lua-format) and normalizes Dhall formatting.
• Updates CI to run a content-based formatting check (nix fmt && git diff --exit-code) alongside existing build/lint steps.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
treefmt.nix	Defines the treefmt-nix formatting module (nixfmt, dhall, purs-tidy, lua-format, excludes).
flake.nix	Wires treefmt-nix into the flake outputs and adds a devShell hook to install a pre-commit formatter check.
.github/workflows/ci.yml	Adds a formatting check step and updates luacheck flags.
.tidyrc.json	Adds purs-tidy configuration for formatting PureScript sources.
.lua-format	Replaces LuaFormatter config with the ecosystem canonical settings and updated line budget.
AGENTS.md	Documents formatting workflow (nix fmt + diff-based check) and updates lint command.
packages.dhall	Normalizes formatting output (removes trailing whitespace line).
flake.lock	Adds the treefmt-nix input pin.

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