Add relevance signals for Box plugin suggestions

[hbkwong]

This PR adds a relevance block to the box entry in .claude-plugin/marketplace.json. It lets Claude Code proactively suggest installing the Box plugin when a session shows Box-related activity, instead of relying on users to find it.

The suggestion fires when, during a session, the agent:

• Runs the box CLI or hits a Box host (api.box.com, upload.box.com, dl.boxcloud.com, account.box.com)
• Reads a manifest depending on a supported Box SDK (Node/TS, Python, Java, .NET, iOS)
• Opens the Box CLI config dir (.box/)

Notes

• Signals are activity-based, not prompt-text based.
• Only surfaces in Claude Code, and only when the marketplace is allowlisted in managed settings.
• Passes claude plugin validate.

[alvinlhan]

What does this signal check for exactly? If claude code reads a file in the .box directory? I looked in my own .box directory and all I saw was a settings.json file, what scenarios would we expect this to trigger?