feat: add --sort-order to milestone update

[bendrucker]

Add --sort-order <value> flag to linear milestone update, exposing the sortOrder field that was already available in the GraphQL schema's ProjectMilestoneUpdateInput but not as a CLI option.

Changes

• Add --sort-order <value:number> option to the update command
• Include sortOrder in the mutation response and display it in output
• Use == null / != null checks (not truthiness) so --sort-order 0 works correctly
• Update validation error message to list the new flag

Testing

• Snapshot test for --sort-order 5
• Dedicated edge case test for --sort-order 0 to guard against truthiness-check regressions
• Updated existing mock responses to include sortOrder field

[schpet]

@bendrucker appreciate it ✌🏻