feat: GitLab MR creation in /ship + /document-release

Ship Step 1.5 now checks .gitlab-ci.yml for release workflows alongside
GitHub Actions. Step 8 routes to glab mr create on GitLab repos with
correct flag mapping (-b, -t, -d). Falls back to manual instructions
when no CLI is available. Document-release now reads MR body via
glab mr view -F json and updates via glab mr update on GitLab repos.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

document-release/SKILL.md.tmpl

@@ -291,14 +291,20 @@ EOF
 git push
 ```
 
-**PR body update (idempotent, race-safe):**
+**PR/MR body update (idempotent, race-safe):**
 
-1. Read the existing PR body into a PID-unique tempfile:
+1. Read the existing PR/MR body into a PID-unique tempfile (use the platform detected in Step 0):
 
+**If GitHub:**
 ```bash
 gh pr view --json body -q .body > /tmp/gstack-pr-body-$$.md
 ```
 
+**If GitLab:**
+```bash
+glab mr view -F json 2>/dev/null | python3 -c "import sys,json; print(json.load(sys.stdin).get('description',''))" > /tmp/gstack-pr-body-$$.md
+```
+
 2. If the tempfile already contains a `## Documentation` section, replace that section with the
    updated content. If it does not contain one, append a `## Documentation` section at the end.
 
@@ -308,18 +314,24 @@ gh pr view --json body -q .body > /tmp/gstack-pr-body-$$.md
 
 4. Write the updated body back:
 
+**If GitHub:**
 ```bash
 gh pr edit --body-file /tmp/gstack-pr-body-$$.md
 ```
 
+**If GitLab:**
+```bash
+glab mr update -d "$(cat /tmp/gstack-pr-body-$$.md)"
+```
+
 5. Clean up the tempfile:
 
 ```bash
 rm -f /tmp/gstack-pr-body-$$.md
 ```
 
-6. If `gh pr view` fails (no PR exists): skip with message "No PR found — skipping body update."
-7. If `gh pr edit` fails: warn "Could not update PR body — documentation changes are in the
+6. If `gh pr view` / `glab mr view` fails (no PR/MR exists): skip with message "No PR/MR found — skipping body update."
+7. If `gh pr edit` / `glab mr update` fails: warn "Could not update PR/MR body — documentation changes are in the
    commit." and continue.
 
 **Structured doc health summary (final output):**