Walks marketplace.json for vendored plugins, extracts http/sse MCP
server URLs from .mcp.json / mcp.json / plugin.json, and probes each
with HEAD then a JSON-RPC POST fallback. Fails on 404/410 and
connection errors; passes on auth/method errors (expected without
credentials). Runs on PR, daily schedule, and manual dispatch.
External (SHA-pinned) plugins are out of scope — their .mcp.json
isn't checked out here.
