feat(resolver,graph): fetch and resolve include: remote: HTTPS URLs

Remote includes (include: remote: https://...) were previously skipped
silently in the resolver and rendered as unexpanded leaf nodes in the
graph.

Changes:
- fetcher.FetchURL: new shared unauthenticated HTTP GET helper
- resolver: resolveRemoteInclude fetches the URL, parses YAML, sets job
  origin to the URL string, recursively resolves sub-includes, and emits
  a warning on failure (lint continues on the rest of the pipeline)
- graph: recurseRemote fetches the URL, captures direct job names, and
  recurses into sub-includes so remote nodes expand like local ones

Adds testdata/includes_remote.yml fixture.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

testdata/includes_remote.yml

@@ -0,0 +1,14 @@
+---
+# Exercises include: remote: URL fetching and sub-include recursion.
+# The remote file is a real public GitLab CI template; it may contain its own
+# includes which should also be resolved. Exit code is 0 (warnings allowed).
+stages:
+  - test
+
+include:
+  - remote: https://gitlab.com/gitlab-org/gitlab/-/raw/master/lib/gitlab/ci/templates/Bash.gitlab-ci.yml
+
+local-job:
+  stage: test
+  script:
+    - echo "local job alongside remote include"