Fix PR status flicker when GitHub returns UNKNOWN mergeable

[Alex-ai-future]

Problem

When refreshing PR status in the sidebar, the PR badge briefly flashes red "Blocked" before settling on the correct state (e.g. green "Mergeable").

Root Cause

GitHub GraphQL API asynchronously calculates the mergeable field. When the calculation hasn't finished yet, it returns "UNKNOWN". The existing PullRequestMergeReadiness code falls through to .blocked for any value that isn't "MERGEABLE" or "CONFLICTING":

if mergeable == "MERGEABLE" {
  self.blockingReason = nil
  return
}
self.blockingReason = .blocked  // ← UNKNOWN lands here

This means every refresh where GitHub hasn't finished computing mergeable temporarily shows "Blocked".

PR #538 fixed a separate flicker issue (confirmed-no-PR clearing), but did not address the UNKNOWN → .blocked path.

Fix

Preserve the last-known mergeable and mergeStateStatus values at the reducer level when the new value is UNKNOWN. All other fields (title, checks, commits, etc.) update normally — only the mergeable state is carried over from the previous PR data.

This is applied in repositoryPullRequestsLoaded before the equality check, so the UI never sees the intermediate UNKNOWN state.

Tests

4 new test cases in RepositoriesFeatureTests:

Test	Scenario
PreservesMergeableWhenNewIsUnknown	New PR has UNKNOWN, old has MERGEABLE → noop (preserved)
PreservesMergeableButUpdatesOtherFields	New PR has UNKNOWN + new title → mergeable preserved, title updated
KeepsUnknownWhenNoPreviousPR	First load with UNKNOWN → stays UNKNOWN (no old value to preserve)
KeepsUnknownWhenPreviousAlsoUnknown	Both old and new are UNKNOWN → stays UNKNOWN (no known value to preserve)

[onevcat]

Reviewed — confirmed the flicker path on main: GitHub GraphQL returns mergeable: UNKNOWN while recomputing (exactly after pushes, when Prowl refreshes), and PullRequestMergeReadiness falls through to .blocked for it. Preserving the previous known value before the equality check is the right layer, and it composes cleanly with the tri-state work from #538 rather than overlapping it. Tests look great too.

Two optional follow-ups if you're interested (both non-blocking):

• The workspace-children path (applyWorkspaceChildrenInfo in RepositoriesFeature+WorkspaceChildren.swift) still writes UNKNOWN directly, so child rows can flicker on their (slower) refresh cadence.
• A previous.number == pullRequest.number guard would avoid grafting the old PR's mergeable state onto a brand-new PR for one cycle in the closed-then-reopened edge case.

Thanks for the careful work! 🙌