Desktop app shows stale PR link after branch switch (polling cache not invalidated)

Description

When switching branches in a git repo, the Claude Code desktop app continues to display a stale PR link from the previous branch for up to ~60 seconds. The session-pr-store cache in Electron Local Storage is not invalidated on branch switch — it waits for the next gh pr view background poll cycle to update.

Steps to Reproduce

1. Have a repo with a feature branch (e.g., dev) that has a merged PR (e.g., PR #122 targeting main)
2. Start Claude Code desktop while on the dev branch — the footer correctly shows main ← dev with a "View PR" link to PR #122
3. Switch to main in your terminal/IDE
4. Immediately open a new Claude Code conversation

Expected: No PR link shown (since main has no associated PR — gh pr view returns "no pull requests found for branch main")

Actual: The footer still shows the old PR link (displayed as main ← main with PR #122) for up to ~60 seconds until the next background gh pr view poll fires and clears it

Evidence

• Running gh pr view manually on main immediately returns "no pull requests found for branch main"
• Running gh pr view on dev returns the merged PR #122
• The Electron Local Storage leveldb (~/Library/Application Support/Claude/Local Storage/leveldb/) contains a session-pr-store that caches branch→PR mappings and is not cleared on branch switch
• The stale PR persists long enough that a user can switch branches, open a new conversation, type a message, take a screenshot, and share it — all while seeing the wrong PR

Environment

• Claude Code Desktop v2.1.51
• macOS (Darwin 24.6.0, arm64)
• gh CLI installed and authenticated

Suggested Fix

Invalidate the session-pr-store cache entry immediately when the detected git branch changes, rather than waiting for the next 60-second gh pr view poll cycle. A branch switch event should trigger an immediate re-poll or cache clear.

Related Issues

• #23580 — gh pr view polling bypasses permission system and cannot be disabled
• #11176 — PR metadata is immutable once set in a conversation