fix: harden hook fallback and MCP node resolution

[Shockang]

Summary

• prefer installed cache plugin paths in plugin/hooks/hooks.json before falling back to the marketplace source path
• resolve the Node.js executable for MCP startup from CLAUDE_MEM_NODE_PATH, common install locations, or PATH instead of hardcoding node
• add regression tests for the hook fallback and runtime resolver changes

Issue Mapping

• Fixes fix(#1215): PR #1229 fallback path points to marketplace source, not installed cache — hooks still fail when CLAUDE_PLUGIN_ROOT unset #1242
• Addresses Worker silently fails init when 'node' not in PATH, reports status:ok but stays initialized:false forever #1289

Test Plan

• bun test tests/infrastructure/plugin-distribution.test.ts tests/infrastructure/process-manager.test.ts tests/worker-spawn.test.ts tests/hook-lifecycle.test.ts tests/server/server.test.ts
• npm run build
• shell-validated the new hooks.json fallback snippet against both cache-present and cache-missing HOME layouts