The target branch picker makes a full GraphQL paginated fetch on every open, causing noticeable delay. Branches should appear instantly on subsequent opens, and pre-fetching should warm the cache before the user even clicks.

Changes

• In-memory branch cache in GitHubRepository: listBranches() caches results for unprefixed (full list) queries. getCachedBranches() provides synchronous retrieval.
• Two-phase quickpick population: changeRemoteAndBranch() now shows cached branch picks immediately, then refreshes in the background with fresh API data.
• Pre-fetch on view init: getCreateParams() fires a non-blocking listBranches() call for the base remote so the cache is warm by the time the user opens the picker.
• Extracted buildBranchPickItems helper in quickPicks.ts shared by both branchPicks() (async, fetches) and cachedBranchPicks() (sync, cache-only).

Flow

Create PR view opens
  └─ getCreateParams() → prefetchBranches() fires in background
                            └─ listBranches() populates _branchesCache

User clicks base branch picker
  └─ cachedBranchPicks() → instant display from cache
  └─ updateItems() → background refresh with fresh API data

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