Fix CLI metadata.generator to report CLI identity instead of plugin identity

[nathanacurtis]

Problem

When generating specs with the CLI, the metadata.generator field in the output incorrectly identifies the generator as the Figma plugin (reporting the plugin version and URL) rather than the CLI. Consumers of spec files cannot tell whether a spec was produced by the plugin or the CLI.

Solution

The CLI should populate metadata.generator with its own identity — the CLI version number and the CLI's package URL — distinct from the plugin's generator metadata.

Acceptance criteria

• Specs generated by the CLI report a metadata.generator that identifies the CLI (correct version, correct URL)
• Specs generated by the CLI no longer report the plugin version or plugin URL in metadata.generator
• Specs generated by the Figma plugin are unaffected

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Implementation details are tracked internally.