feat(notifier): gitlab_artifact notifier (Wave 1 Tier-2 #1)

[zuchka]

Summary

Adds a built-in `type: gitlab_artifact` notifier that writes alert Markdown to a configurable file path (default `ding-alerts.md`, relative to `$CI_PROJECT_DIR`), so GitLab CI users can declare it in `.gitlab-ci.yml` `artifacts:` and surface alerts as a downloadable pipeline artifact in the GitLab UI. Closes the gap flagged in the GitLab CI recipe — first row of the Wave 1 Tier-2 candidates table marked shipped.

This continues the established Tier-2 cadence: T2-A (`${VAR}`), T2-B (`kubernetes_event` shipped in v0.9.0), and now Wave 1 #1 (`gitlab_artifact`).

Design (locked in brainstorm)

• Format: Markdown only (mirrors GHA notifier shape). No `format:` config field.
• Path: optional `path:` field; defaults to `ding-alerts.md` in CWD. Multiple notifier instances → multiple files.
• Pattern: sync, mutex-guarded, append-only. Mirrors `internal/notifier/github_actions.go` exactly minus the `::warning::` stdout mirror (GitLab CI lacks workflow-annotation commands).
• No CI gate: writes the file regardless of whether running in GitLab CI. Outside CI, it just makes a local file — harmless.
• No async queue / retry / metrics: sync local file write; nothing to instrument.

Commits

1. `feat(notifier): gitlab_artifact notifier (sync Markdown append)` — impl + 7 tests + 1 new field on `NotifierConfig` + config + server registry wiring (303 insertions, no new deps)
2. `docs(recipes): document gitlab_artifact notifier` — recipe positive section, configuration.md entry, ding.yaml.example block

Test plan

• `go test -race ./...` — full suite green
• `go test -race ./internal/notifier/... -run TestGitLabArtifact -v` — all 7 new tests pass
• `go build -o /tmp/ding ./cmd/ding && /tmp/ding --help` — binary builds
• Local smoke (light, no real GitLab CI required):
  ```bash
  mkdir /tmp/gitlab-artifact-smoke && cd /tmp/gitlab-artifact-smoke
  cat > ding.yaml <<'YAML'
  notifiers:
  alerts:
  type: gitlab_artifact
  rules:
  • name: job_failed
    match: { metric: run.exit }
    condition: value > 0
    message: "Job failed (exit {{ .exit_code }})"
    alert:
    • notifier: alerts
      YAML
      /tmp/ding run --config ding.yaml -- false
      cat ding-alerts.md

  Expected: file with "# DING Alerts" header + "## job_failed" section

  ```
• mkdocs build --strict (CI runs it; not run locally if mkdocs isn't installed)

Out of scope (deferred)

• `format: json | jsonl | plain` field — Markdown only for v1
• `artifacts:reports:*` integration — repurposes report schemas; complexity > value
• Generic `markdown_file` notifier — keep `gitlab_artifact` per Wave 1 spec naming
• Path safety guards — YAGNI

Post-merge

By the established convention (MLflow → v0.6.0, Argo → v0.7.0, Ray → v0.8.0, kubernetes_event → v0.9.0), this could tag as v0.10.0 if you want the new notifier in a release rather than just on main.

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