docs(cli): add sandbox command usage examples

[luojiyin1987]

Summary

• add a Common sandbox commands section to the CLI README
• document common sandbox workflows with concrete examples
• keep this guidance in docs instead of expanding command-level help text

Changes

• document sandbox create, list, info, connect, logs, metrics, and exec
• include examples for --detach, state and metadata filters, JSON output, and --follow
• clarify sandbox exec usage with --, environment variables, and piped stdin

Closes #1289

[claude]

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