feat(cmd): release v1.10.0 — shallow clone guard and --check preflight

- refuse to release when the clone is shallow and no previous release tag
  is found: tags beyond the fetch depth would silently restart versioning
  at X.Y.0; --allow-shallow bypasses the guard for a true first release
- new --check preflight validates the release environment without
  releasing: branch resolution + pattern, working tree, tag discovery,
  shallow clone, remote origin URL parse (same parse the push performs),
  push auth method, version files, release target — all problems reported
  at once, non-zero exit on failure
- .releaser.gitlab-ci.yml gains an optional .releaser:check MR-pipeline
  job; CI examples now set GIT_DEPTH: 0

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.releaser.gitlab-ci.yml

@@ -20,6 +20,19 @@
 #                   api + write_repository
 #                   (do NOT use CI_JOB_TOKEN — it cannot push tags back)
 
+# Optional preflight job for merge-request pipelines: validates the release
+# environment (branch pattern, remote URL, auth, version files, release
+# target) without releasing anything.
+.releaser:check:
+  stage: test
+  image: ${RELEASER_IMAGE:-registry.example.com/releaser:latest}
+  variables:
+    GIT_DEPTH: 0
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+  script:
+    - releaser --check --branch "release/0.0"
+
 .releaser:release:
   stage: release
   image: ${RELEASER_IMAGE:-registry.example.com/releaser:latest}