Add nimvault MCP plugin

[HaoZeke]

Summary

Adds third-party plugin nimvault: GPG opaque-blob vault MCP (list/status/scan; gated mutate). Local CLI + optional libnimvault in-process path.

• Source: https://github.com/HaoZeke/nimvault-mcp (MIT)
• Pinned SHA: 66344b53ba9ad95b7f77d09a4e149e2be9414bf0 (v0.3.3)
• CLI/lib: nimvault v0.4.2 (nimble buildLib / NIMVAULT_LIB for in-process)
• MCP releases: https://github.com/HaoZeke/nimvault-mcp/releases/tag/v0.3.3 (also v0.3.2 Unix assets)
• Mutating tools require NIMVAULT_MCP_ALLOW_MUTATE=1
• Includes skill + plugin.json + .mcp.json

Checklist

• Pin full commit SHA (0.3.3 / 66344b5…)
• Regenerated plugin-index.json
• validate-catalog.py passes locally
• Keywords brand-scoped (nimvault)

Test plan

• grok plugin install from marketplace after merge
• nimvault_doctor shows CLI + optional libnimvault line
• nimvault_status with repo_path set