Add milestone support to dn issue create

[bpmooch]

Current State

dn issue create supports title, repository, body, labels, and JSON output, but it cannot assign the new issue to a GitHub milestone.

This was confirmed against the current main implementation in cli/issue.ts: the create argument parser has no milestone option, the help text does not advertise one, and the constructed CreateIssueOptions contains only title, body, and labels.

As a result, workflows that otherwise use dn issue create must fall back to gh or a direct GitHub API request when creating milestone-scoped work.

Expected State

Add milestone assignment to dn issue create.

Suggested interface:

dn issue create --title "Build operator" --milestone "denoise infra launch!"
dn issue create --title "Build operator" --milestone 17

Acceptance criteria:

• dn issue create accepts --milestone <title|number>
• A numeric value resolves to the repository milestone number
• A title resolves against open milestones in the target repository
• Missing or ambiguous milestone titles fail with an actionable error before issue creation
• The resolved milestone is passed through the public create-issue SDK options
• --repo owner/repo resolves milestones in the target repository, not the current repository
• Omitting --milestone preserves current behavior
• CLI help and examples document milestone assignment
• Tests cover numeric IDs, exact titles, missing milestones, ambiguous titles, cross-repository creation, and omission

Additional Context

This was encountered while creating an epic and subtasks in chesapeakedev/chesapeake under the denoise infra launch! milestone. Native sub-issue relationships were handled successfully through dn issue relationship, but milestone assignment required gh.