On Android the header shows one action and folds the rest into the
three-dots menu, which had no entry for the thread's linked PRs. Add one:
it opens the only linked PR directly, or the Git sheet's list when several
are linked.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Signed-off-by: Tiger Kaovilai <passawit.kaovilai@gmail.com>