docs: add metric-commands.md (task 8i)

Document all metric subcommands with real CLI output examples:
- metric show: inspect metric info for top of stack
- metric list: list all metric categories
- metric <category>: list units in a category
- metric compatible: check compatibility of two values
- metric decimal set / metric binary set: SI vs IEC modes

Covers REPL vs single-command differences, SI vs IEC prefix modes,
and practical use cases for exploring and verifying metrics.

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