feat: add --json output

[SafeEval]

lint.Problem already carried json:"rule" and json:"message" tags, but nothing ever emitted them. A linter that runs in CI should be able to hand its results to something other than a human, so this finishes the API the types already declared.

$ commitlint lint --json "Add gateway"
{
  "conforms": false,
  "checked": 1,
  "failed": 1,
  "blocking": true,
  "results": [
    {
      "label": "message",
      "subject": "Add gateway",
      "problems": [
        {
          "rule": "format",
          "message": "subject must be `type(scope)?: description`, got \"Add gateway\""
        }
      ]
    }
  ]
}

Design

Results are collected into a Report and rendered as either text or JSON, so both formats derive from one structure and can't disagree about what was found or what the exit code should be.

Two details worth calling out:

• blocking is separate from failed. In --mode warn a run has problems but doesn't fail. A consumer needs to distinguish "found problems" from "failed the run", and failed > 0 alone can't express that.
• problems is always [], never null. Callers can iterate without a nil check.

Verification

• Text output is byte-for-byte unchanged, diffed against a binary built from merged main (1c3a0df) across conforming, violating, warn-mode, and multi-problem-file cases. This was the main regression risk in the refactor.
• JSON validated by parsing with a real JSON reader (not eyeballing), for conforming, violating, warn-mode, and multi-message --range runs.
• The README example is copied from actual output and matches byte for byte.
• First tests for package main, which previously had none — covering the JSON shape as a CLI contract, the []-not-null guarantee, and all three text renderings.
• gofmt, go vet, go test -race ./... clean. Zero third-party dependencies preserved (encoding/json is stdlib).

Release impact

This is a feat:, so merging cuts v1.1.0 — not the v1.0.1 discussed earlier. That's the automation working as designed: the version follows the change type. The comment-stripping fix from #1 ships in v1.1.0 along with this.

This will also be the first live exercise of release.yml via the merge-to-main path.

🤖 Generated with Claude Code

https://claude.ai/code/session_01VSrrciEDBTuFNMtKocScML