feat: add terminate command to terminate a dataset

[akronim26]

Fixes #189

[Copilot]

Pull request overview

This pull request adds a new terminate command to the data-set CLI that allows users to terminate a dataset and its associated payment rails. This addresses issue #189 which requested a convenient way to cleanup and stop paying utility for datasets.

Changes:

• Added a new terminate subcommand to the data-set command that terminates both the dataset and its payment rails
• Implemented ownership validation to ensure only the dataset owner can terminate
• Added idempotent handling for already-terminated datasets
• Enhanced piece display to show terminated datasets' pieces as "onchain orphaned"

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/commands/data-set.ts	Adds the new terminate subcommand registration with argument parsing and error handling
src/data-set/run.ts	Implements runTerminateDataSetCommand with owner validation, confirmation prompt, transaction submission, and result display
src/data-set/display.ts	Updates renderPieces to mark pieces as ONCHAIN_ORPHANED when dataset's payment rail is terminated
src/test/unit/data-set.test.ts	Adds comprehensive test coverage for successful termination, permission denial, and already-terminated scenarios

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