feat(terminal): add confirmation defaults and fix form input loss

Every confirmation defaulted to No, so Enter after a filled-in form
canceled it; prompts were titled with raw flag names and aborted on an
empty answer; and the shared issue and pull request form lost input:

- Multi-selects hid their last option, so a single assignee looked like
  an empty list and labels showed four of five. They now get an explicit
  height and scroll past eight options.
- A pty without a window size, such as `script` run without a terminal,
  made bubbles panic. Forms now refuse it with "terminal size unknown;
  pass the values as flags or use --no-input".
- Without a configured editor, the form ran to the end and then failed
  with "configure an editor", discarding everything typed. The editor is
  now resolved before the form; without one the body is typed into an
  inline multi-line field.
- The pull request form said "Issue title" and "Issue description". The
  shared fields read "Title (required):" and "Body (markdown):", and the
  review body label gained its missing space.

New prompt API for the area commands: confirmAction with a
terminal.Confirmation (DefaultYes for constructive actions, Typed for
type-to-confirm, NonTTY to name the action in the --yes error) and
requireInput with a human label and inline validation. Declined or
aborted prompts return terminal.ErrCanceled (exit code 3). The existing
confirm and require helpers keep working on top of them, and prompters
without the new terminal.Asker methods keep the plain behavior.

Signed-off-by: bircni <bircni@icloud.com>

internal/terminal/pull_form_test.go

@@ -35,7 +35,7 @@ func TestPullFormMarksRequiredFields(t *testing.T) {
 		{
 			"without head", "n\nmain\nalice\nfeature\ny\nTitle\n",
 			PullFormChoices{},
-			[]string{"Target branch (required):", "Source branch (required):", "Issue title (required):"},
+			[]string{"Target branch (required):", "Source branch (required):", "Title (required):"},
 			nil,
 		},
 		{