#28276 carry of #27675: add PlatformMachines snapshot deletion

Carry of #27675: add PlatformMachines snapshot deletion. This branch merges current feat-cmux-next and carries the original purpose and authorship while repairing snapshot ownership expiry and failed-delete retry findings.

Original PR: https://github.com/manaflow-ai/cmux/pull/27675

Agent session: agent session

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Summary by cubic

Adds the deleteSnapshot RPC to PlatformMachines so a user can delete their own platform-created VM snapshots, including after the snapshot's creation ledger has been pruned and after the user has left the team.

• Platform snapshot rows now carry a platform marker that survives ledger pruning and source-machine deletion; legacy platform snapshots without the marker get the marker backfilled in the same transaction that prunes their creation key.
• Failed provider deletes are retried idempotently instead of being trapped forever; deleting an already-deleted snapshot reports success.
• Deleting a non-platform snapshot is refused without calling the provider.

Migration

• No data migration needed; legacy platform snapshots are handled automatically the next time their creation ledger is pruned.