Keep the sidebar's open rows across refreshes

Expanding a workflow run and waiting through one poll closed it again.

Two causes. Nova applies whatever collapsible state getTreeItem returns,
and it returned Collapsed for every row that was not built default-open,
so each reload closed everything the user had opened. On top of that, a
reload fired on every poll whether or not anything had changed, so an
idle tree was rebuilt every 15 to 60 seconds for no reason.

The provider now tracks expansion by node identifier, fed from the tree
view's expand and collapse events. A node built default-open seeds that
set the first time its identifier is seen, so such a section can still be
closed by hand and stay closed.

Node identifiers consequently have to be stable across rebuilds. The
fallback for nodes without an explicit one was a counter, which changed
on every build; it is now derived from the node's kind and name. Step
rows were keyed by step number alone, which collided between jobs, and
are now qualified by the job.

Reloads are also skipped entirely when store.signature() is unchanged.
The signature covers everything drawn, including the branch filter, which
changes the tree without changing any data. A live run contributes a
coarse time bucket so its elapsed clock still ticks, and the explicit
Refresh command always redraws.

Adds twelve checks covering expansion surviving a rebuild, a default-open
section staying closed once closed, and the signature responding to run
state and the branch filter but not to a no-op poll. 143 checks total.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01MQuusXgZC2dzwpJJ1qhtti

CHANGELOG.md

@@ -29,3 +29,5 @@ Initial release.
 - Adaptive polling and a notification when a workflow run fails.
 - Gitea Actions CI running the test suites, script syntax checks, manifest
   validation, and a generated-image check.
+- The sidebar keeps its open rows and scroll position across refreshes, and a
+  poll that changes nothing no longer redraws it at all.