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

[Unisay]

Pilot of the ecosystem formatting setup (ADR 0007). Wires treefmt via treefmt-nix and formats the tree.

Tooling.treefmt.nix runs nixfmt, dhall format, purs-tidy (.tidyrc.json) and LuaFormatter (.lua-format). LuaFormatter is kept over StyLua because it preserves the parentheses pslua's foreign-file parser requires; the FFI keeps its compact one-line style with a 130-column budget, matching the raised luacheck --max-line-length 130.

Wiring.nix fmt formats; the flake exposes checks.formatting; the dev shell installs a pre-commit hook; CI gains a format-check step. The check is content-based (nix fmt && git diff --exit-code) rather than treefmt --ci, because the in-place formatters bump mtime even when content is unchanged, which trips treefmt's --fail-on-change.

Diff. Most of the changes are the first format pass over *.purs and the *.lua FFI. Verified locally: build, regression test, luacheck --max-line-length 130, and the content-based format check all pass.

This is the template for rolling the same setup across the other forks + compiler.

[Copilot]

Pull request overview

This PR pilots an ecosystem-wide formatting setup by integrating treefmt (via treefmt-nix) into the flake, wiring nix fmt as the unified formatter entrypoint, and enforcing formatting via a content-based check in CI (plus an installed pre-commit hook in the dev shell).

Changes:

• Add treefmt.nix configuration and expose formatter / checks.formatting from the flake.
• Add CI “Format check” step (nix fmt && git diff --exit-code) and document formatting/linting commands.
• Apply an initial formatting pass across PureScript and Lua (FFI) sources and configs.

Reviewed changes

Copilot reviewed 11 out of 14 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
treefmt.nix	Defines tree-wide formatting rules (nixfmt, dhall, purs-tidy, lua-format) and excludes generated/vendor directories.
flake.nix	Adds treefmt-nix, exposes formatter + formatting check, and installs a pre-commit hook from the dev shell.
flake.lock	Locks the new treefmt-nix input.
.github/workflows/ci.yml	Adds a content-based formatting verification step in CI.
AGENTS.md	Documents the new nix fmt workflow and updated luacheck line-length setting.
.tidyrc.json	Adds purs-tidy configuration for PureScript formatting.
.lua-format	Adds LuaFormatter configuration for FFI formatting.
.gitignore	Ensures the new dotfiles are tracked despite the broad dotfile ignore rule.
spago.dhall	Formatting cleanup (whitespace).
src/Effect/Uncurried.purs	PureScript formatting (layout/line breaks) for readability and consistency.
src/Effect/Uncurried.lua	Lua FFI formatting (compact style consistent with formatter config).
src/Effect.purs	PureScript formatting adjustments (exports + foreign import layout).
src/Effect.lua	Lua formatting (compact style).
test/regression/uncurried.lua	Test formatting adjustments.

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