Move the external agentic-qe-fleet entry from v3.15.16 (4edfa469) to
v3.15.17 (b177b2315521376b83f2eed40e4994fd7581d067), so its MCP server
runs agentic-qe@3.15.17. The plugin folder changed only its version and
MCP pin. Updates the README links, the external-plugins test fixture and
the ADR-484 evidence section with the re-run install results.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>