Add Lakebox CLI for managing Databricks sandbox environments

[shuochen0311]

Summary

Lakebox provides SSH-accessible development environments backed by microVM isolation. This adds CLI commands for lifecycle management.

Commands

• lakebox auth login — authenticate to a Databricks workspace
• lakebox create — create a new lakebox (with optional SSH public key)
• lakebox list — list your lakeboxes (shows status, key hash, default)
• lakebox ssh — SSH to your default lakebox (or create one on first use)
• lakebox status <id> — show lakebox details
• lakebox delete <id> — delete a lakebox
• lakebox set-default <id> — change the default lakebox

Features

• Default lakebox management stored at ~/.databricks/lakebox.json per profile
• Automatic ~/.ssh/config management (single block, in-place update)
• Public key auth only (password/keyboard-interactive disabled in SSH config)
• Creates and sets default on first lakebox ssh if none exists
• Talks to the lakebox REST API at /api/2.0/lakebox on the workspace host

Test plan

• Tested e2e against dbsql-dev-testing-default.dev.databricks.com
• Create, list, status, delete, ssh all working
• Default management (set on create, clear on delete, reuse on ssh)

This PR was created by the Lakebox team (Infra2.0/Brickvisor).

[github-actions]

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• PR number: 4930
• Commit SHA: 756dfa2ebf729269206662d60e64370463bc1945

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[github-actions]

Waiting for approval

Based on git history, these people are best suited to review:

• @kanterov -- recent work in cmd/
• @pietern -- recent work in cmd/
• @anton-107 -- recent work in cmd/

Eligible reviewers: @andrewnester, @denik, @shreyas-goenka, @simonfaltum

_{Suggestions based on git history. See OWNERS for ownership rules.}