feat(huh prompter): add placeholder to search input

Add 'Type to search, Ctrl+U to clear' placeholder to the
MultiSelectWithSearch search input. Set WithWidth(80) in the test
harness to prevent textinput placeholder rendering panics when
there is no terminal.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/prompter/huh_prompter.go

@@ -227,6 +227,7 @@ func (p *huhPrompter) buildMultiSelectWithSearchForm(prompt, searchPrompt string
 		huh.NewGroup(
 			huh.NewInput().
 				Title(searchPrompt).
+				Placeholder("Type to search, Ctrl+U to clear").
 				Accessor(queryAccessor),
 			huh.NewMultiSelect[string]().
 				Title(prompt).

internal/prompter/huh_prompter_test.go

@@ -113,7 +113,7 @@ func newTestHuhPrompter() *huhPrompter {
 func runForm(t *testing.T, f *huh.Form, ix interaction) {
 	t.Helper()
 	r, w := io.Pipe()
-	f.WithInput(r).WithOutput(io.Discard)
+	f.WithInput(r).WithOutput(io.Discard).WithWidth(80)
 
 	errCh := make(chan error, 1)
 	go func() { errCh <- f.Run() }()