Resolve milestone by name on gitea issue Create/Update

[andrew]

Fixes #76.

forge issue create -m <milestone> and forge issue edit <n> -m <milestone> were silent no-ops against Gitea/Forgejo because giteaIssueService.Create and Update never read opts.Milestone. Same class of bug as #75.

Unlike labels, gitea.CreateIssueOption and gitea.EditIssueOption already have a Milestone field, so this goes through the existing calls rather than a separate endpoint. The SDK provides GetMilestoneByName so the title-to-ID resolver is a single lookup rather than a paginated scan.

Empty string on Update sends milestone: 0 to clear the assignment. An unknown title produces milestone not found: <name> instead of silently succeeding.

The Update branch is placed before the existing labels block so that setting both -l and -m together still reaches EditIssue after ReplaceIssueLabels fires.