feat(validator): add CUE file validation support

[mvanhorn]

Summary

Adds syntax validation for CUE (.cue) files using the official cuelang.org/go SDK. CUE is gaining traction in the Kubernetes / DevOps ecosystem (Dagger, KubeVela), and the issue specifically calls out the pure-Go SDK as the right entry point.

What changed

• pkg/validator/cue.go — new file. CueValidator wraps cue/parser.ParseFile and returns ValidationError{Err, Line, Column} when the parse fails. Mirrors the position-extraction pattern in TomlValidator (pkg/validator/toml.go:11).
• pkg/filetype/file_type.go — new CueFileType (extension cue), registered in fileTypeRegistry and added to the FileTypes slice in init() so the file is dispatched to the new validator.
• pkg/validator/validator_test.go — validCue and invalidCue cases added to the existing testData slice.
• .pre-commit-hooks.yaml — cue added to both types_or and the files regex on the config-file-validator hook so that changed .cue files actually invoke the validator under the standard pre-commit integration. (Without this, the hook's type filter would silently skip .cue files even though the validator is registered.)
• go.mod / go.sum — adds cuelang.org/go and its transitive dependencies via go get + go mod tidy.

68 lines added, 19 removed (the bulk of go.sum accounts for the rest).

Verification

• go build ./... — pass
• go vet ./... — pass
• go test ./pkg/validator/... ./pkg/filetype/... — pass

Scope notes

• Schema validation deferred. CUE has a built-in constraint system; that can land as a follow-up implementing SchemaValidator on CueValidator. This PR is the syntax-only foundation called out in the issue.

Closes #462

This contribution was developed with AI assistance.

[kehoecj]

@mvanhorn Please try to limit the amount of PR's to one or two at a time. Especially when the previous ones have undressed issues.

[mvanhorn]

Fair point, apologies. I'll focus on getting #461 (lint + changelog) and #464 (changelog) cleaned up first, and hold off on new PRs to this repo until those land.

[kehoecj]

@mvanhorn Implementation is solid and passed against a set of real cue files. A few things to clean up before merging (I'm going to assume KDL goes before this one):

• CHANGELOG entry
• Rebase. Your branch is downgrading several dependencies because it's based off an older main. After rebasing, run go get cuelang.org/go@v0.16.1 then go mod tidy. This will also pull in my docs update.
• The current dependency is a dev pre-release (v0.17.0-0.dev.0.20260319...). I checked and v0.16.1 (latest stable) has the same parser.ParseFile API. Please use cuelang.org/go v0.16.1 instead.
• Docs updates:
  • website/docs/reference/supported-file-types.md: Add cue
  • website/docs/introduction.md: update format count from 17 -> 18
  • README.md: update format count from 17 -> 18 and add cue

[mvanhorn]

Thanks @kehoecj. Pushed a650a72:

1. Rebased on current main (go.sum conflict resolved).
2. Switched cuelang.org/go from v0.17.0-0.dev.0 to v0.16.1. parser.ParseFile API is identical; tests pass. go mod tidy applied.
3. CHANGELOG entry under Unreleased / Added.
4. README format count bumped from 16 to 17. Note: assumes this PR lands before KDL feat: add KDL file validation support #464; if KDL lands first the count will already be 17 and this needs to be 17->18 instead. Happy to rebase on feat: add KDL file validation support #464 if you want them stacked.

The website/ docs paths in the review (supported-file-types.md, introduction.md) don't exist in this repo - looks like that's published from elsewhere or planned but not yet in tree. Let me know if I should add anything else.

go test ./..., go vet ./..., and golangci-lint run ./pkg/validator/... all clean.

[kehoecj]

Suggested change

	Config File Validator validates config files across 17 formats.
	Config File Validator validates config files across 18 formats.

[kehoecj]

@mvanhorn KDL was merged, please resolve conflicts and bump count to 18. Thanks!