v0.5.2 — fix mermaid rendering, stale MCP server, orphan stop

Fixed

• Mermaid diagrams silently skipped. initMermaid set data-processed="true" on every <pre class="mermaid"> element before calling mermaid.run(). Mermaid v11 checks this attribute internally and skips already-processed nodes, so no diagram ever rendered. Removed the premature attribute — Mermaid now manages it after successful SVG injection.
• MCP stale server reuse. getOrCreateServer cached the server promise without re-checking liveness. If the singleton was killed externally, the MCP kept returning a dead URL. It now pings the cached server before reuse and recovers automatically.
• mdprobe stop orphan detection. When no lock file exists, stop now scans ports 3000–3010 via /api/status to find and kill orphaned server instances instead of silently reporting "no server running."
• mdprobe stop stale-lock handling. Cleaning a stale lock then finding no orphan now correctly reports stale-lock-cleaned instead of no-lock, fixing the unit tests that blocked the v0.5.2 publish.

Internal

• Added a version-controlled pre-push git hook (auto-activated via the prepare script) that runs the full test suite before every push.
• Isolated stop-cmd unit tests from the network to remove port-scan flakiness under parallel test runs.