Show the pull request a branch already has

[mtsgrd]

Selecting a branch in the branches explorer showed a Diff / Pull Request toggle whose form could not work: publish_review derives the base from the branch's position in a workspace stack, and an unapplied branch has none.

The pane dispatched on the operand — what is selected, not what it is — and both outline tabs mint a Branch. It now takes a DetailsSelection:

AppliedBranch | UnappliedBranch | Commit | UncommittedFile

WorkspacePage already resolves selection per tab, so it names what it resolved. Applied and unapplied are the words the rest of the app uses — unappliedStacks is what fills the branches tab, and ListedBranch was already taken by the SDK type those stacks are made of.

What the branches tab shows now

	header	body
branch has a pull request	Diff / Pull Request toggle	diff, or the full review
branch has none	title only	diff

A review is keyed by branch name, so the description, conversation, checks, reviewers and labels were already reachable — nothing new was needed to read them. The toggle appears only alongside a review, so it is never a half-dead control. Opening a review is still not offered here; apply the branch and use the workspace, where the base exists.

AppliedBranchDetails and UnappliedBranchDetails share BranchTabToggle, useBranchTabHotkeys, BranchDiff, BranchTitleRow and ReviewView. Editing the description stays the applied view's affordance; the review's own controls work in both.

Operand keeps its shape. It is the argument type of getOperations, and an unapplied branch is precisely what operations cannot act on — so the distinction belongs where the two tabs converge, not in a dozen exhaustive matches that could not act on it. Its Branch case is now commented to say it means an applied branch, which it already did: operandLabel asserts the ref resolves to a segment.

Verification

Driven against the running dev app over CDP: an unapplied branch with a PR shows both tabs and the full review (description, checks, reviewers, labels, activity); one without shows title and diff only; an applied branch is unchanged and its pull-request tab still loads.

Checked against the committed tree in a scratch worktree, not the working copy: typecheck, oxlint over the whole repo, prettier, knip:prod, knip:non-prod and the lite unit tests (77/77) all clean.

[samhh]

From a UX perspective there are two issues:

1. There is now layout shift when navigating between a branch with a PR and one without one.
2. I might want to define a draft PR, and perform other actions, on a branch that's not applied. I don't personally see the value in this limitation; perhaps it speaks to a wider habit of how we treat the workspace and everything else as two different worlds.