Create arc buildable command

[albertywu]

Add arc buildable command to provide a CLI interface for fetching Harbormaster buildable summaries.

This new command allows users to query build status by Differential revision ID or PHID, calling harbormaster.getbuildablesummary and printing the raw JSON response to stdout.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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