feat(create): add stepwise interactive prompts when no args given

[jamesjohnsdev]

Mirrors UX: prompts for title, body editor,
labels, assignees, and milestone in sequence. Empty or
whitespace-only title aborts without creating a file.

Also adds injectStdin/readMDFiles test helpers, TestCreateInteractive
(9 subtests), and FuzzCreateInteractive covering all prompt fields.

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Summary by cubic

Adds an interactive mode to create when no args are given, prompting for title, optional body (via editor), labels, assignees, and milestone. Also tightens arg handling to create [title] and aborts cleanly on empty titles.

• New Features
  • No-arg run enters prompts; type e to open body in $VISUAL/$EDITOR if set.
  • Empty/whitespace-only title cancels with no file created.
  • Filenames use next T-number and a slug from the title.
  • Accepts at most one arg; with a title arg, opens editor if available; --editor still requires $VISUAL/$EDITOR.

^{Written for commit 7d8c141. Summary will update on new commits.}