Create a new issue in a milestone from a milestone card

[jnwrnr]

User story

Same as the epic-child flow (#TBD) but starting from a milestone card: I click 'Create issue in this milestone' and the new issue is scoped to one of the milestone's projects.

Scope

• Button on milestone cards.
• Same form as standalone creation, pre-filling milestone_id.
• POST /projects/:id/issues with milestone_id set.

Caveat

Group milestones can contain issues from multiple projects, so the form still needs a project picker. Flagged by the user as potentially complex / lower priority than the epic variant. Defer until the epic flow is working.

Acceptance criteria

• 'Create issue in this milestone' action on milestone cards.
• New issue lands in the chosen project with the milestone already attached.
• Works for both project and group milestones.
• Standard error handling + scope messaging.