fix: replace YAML-conflicting markdown separators in release workflow

Changed Markdown horizontal rules from "---" to "___" in heredoc content
to prevent YAML parser conflicts. The "---" string at the start of a line
is a YAML document separator which causes parsing issues even inside bash
heredocs within GitHub Actions workflows.

Root cause:
- Lines 151 and 226 contained "---" Markdown horizontal rules
- YAML parsers interpret "---" as document separators
- This prevented workflow jobs from executing (no jobs scheduled)
- Changed to "___" which creates identical Markdown output

Impact:
- Fixes workflow runs 18804889309, 18804651253, 18802795662
- All runs failed to schedule jobs due to YAML parsing errors
- Both "___" and "---" render as horizontal rules in Markdown
- Workflow should now properly detect version changes and execute all jobs

History of YAML issues in this workflow:
1. Commit 952a97e: Fixed quoted multi-line strings → changed to heredocs
2. Commit 79ef853: Removed emoji from heredoc (UTF-8 encoding issue)
3. This commit: Replace "---" with "___" (YAML document separator issue)

Concieved by Romuald Członkowski - www.aiadvisors.pl/en

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.github/workflows/release.yml

@@ -148,7 +148,7 @@ jobs:
 
 This is the initial release of n8n-mcp v$CURRENT_VERSION.
 
----
+___
 
 **Release Statistics:**
 - Commit count: $(git rev-list --count HEAD)
@@ -223,7 +223,7 @@ EOF
 
           ${{ needs.generate-release-notes.outputs.release-notes }}
 
-          ---
+          ___
 
           ## Installation