chore: enforce formatting with lint checks and git hooks (#55)

[abienkowski]

Description

Part of #55 (comment 3). Formatting is now enforced, both in CI and locally. Before this, make lint-* ran only go vet, cargo check and tsc. On main, gofmt flagged 3 Go files, cargo fmt reported diffs, and prettier (at width 120) flagged 13 TS files.

This PR does not close #55. The remaining items follow in a separate PR.

What changes

• Formatters, one config per language:
  • Go: gofmt.
  • Rust: cargo fmt, with rs/rustfmt.toml = max_width = 120.
  • TS: prettier 3.9.9, pinned exactly as a devDependency, with ts/.prettierrc = { "printWidth": 120 }.
  • Width 120 is closest to the existing style: compared with the defaults, it cuts the one-time diff from 119 to 72 rustfmt hunks, and from 254 to 197 changed TS lines.
• One-time reformat: one style: commit per language, containing formatter output only (see Verification).
• Lint gates:
  • make lint-go fails when gofmt -l lists a file, or when the file doesn't parse, and points to make fmt-go.
  • lint-rs adds cargo fmt --check.
  • lint-ts adds npm run format:check.
  • CI's rust job installs the rustfmt component, and rust-toolchain.toml also requests it, so a minimal local rustup works too.
• Fix targets: make fmt-go, fmt-rs, fmt-ts and fmt-all. fmt-ts and the hook call ts/node_modules/.bin/prettier directly and refuse to run without it, so they can never format with a different prettier version. Documented in CONTRIBUTING and AGENTS.md.
• Opt-in git hooks (make hooks sets core.hooksPath=.githooks; documented in CONTRIBUTING):
  • pre-commit checks the formatting of staged Go, Rust and TS files, in about a second.
    • It reports every failing language, with the make fmt-* command to run and --no-verify for skipping once.
    • It never rewrites files.
    • It fails closed on a missing tool, and on file names git still quotes.
    • Go and TS check the staged files; Rust checks the whole crate (cargo fmt --check), because rustfmt follows mod declarations.
  • pre-push runs make lint-all.

Review notes

• Squash or merge: both are safe. Every commit passes on its own, and no commit has a body or footer.
• Release: merging cuts v0.3.2, a patch, because the release workflow runs on every push to main. A simulated default squash message gives bump=patch tag=v0.3.2. The release would contain no behaviour change.
• Partial staging: the pre-commit check reads the working-tree copy of each staged file, so with git add -p it can disagree with what is committed. Pre-push and CI are the backstop. This is documented in the hook and in CONTRIBUTING.

Verification

• The style: commits are formatter output only. I checked out each commit's parent, re-ran the formatter, and compared the result with the commit's tree. They were identical for 5768631 (gofmt), 694a175 (cargo fmt with the commit's rustfmt.toml) and e17231d (prettier 3.9.9).
• The CI toolchain agrees with the local one. cargo fmt --check under rust:1.85 (rustfmt 1.8.0) shows no diff. Local rustfmt is 1.9.0.
• The gates catch bad formatting. One stray space per language makes make lint-<lang> fail (exit 2).
• The hooks work. I exercised them in a scratch clone with a local bare remote.
  • Refused:
    • misformatted Go, Rust and TS staged together, with all three languages named;
    • misformatted files named a b.go and é.go;
    • commits made from a subdirectory or a git worktree;
    • file names containing " or \;
    • a push of a misformatted commit.
  • Accepted: docs-only commits, formatted commits, merge commits and --no-verify.
  • shellcheck is clean.
• The suites pass with unchanged counts: make test-all gives Go 108, Rust 145, and TS 162 with 1 skipped. make lint-all passes.

Type of change

• Bug fix
• New feature
• Breaking change
• Tooling / maintenance (formatting, lint gates, git hooks)

Implementation(s) changed

• Go (formatting only)
• Rust (formatting only)
• TypeScript (formatting only)
• Quint specification
• CI / infrastructure

Testing

• Unit tests pass (make test-all): Go 108, Rust 145, TS 162 (1 skipped)
• Integration tests pass: not run locally, because the changes are formatter-only; CI runs all three suites
• Quint verification: not affected
• New checks added: make lint-* format gates and the git hooks

Checklist

• I have read CONTRIBUTING.md
• My code follows the project's coding style
• I have updated documentation as needed (CONTRIBUTING: git hooks)