A PR linked to the thread on another ref (a fork branch, another stack
layer) was invisible to the checkout-derived Git status. The header
quick action and the Git sheet offered Create PR instead of View PR, and
the Android header overflow had no way to reach linked PRs.

Git status for the header menu, Git sheet and review menu now falls back
to the thread's current open link when the checkout's ref has no open PR.
Closed and merged links are ignored, and unsynced links count as open.
The Android header gets a linked-PR action that reuses the thread list
badge behavior: it opens a single link and lists several. The linked-PR
list now shows links the thread already has, regardless of server
capability, which only gates editing them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>