Messages submitted while a branch summary was being generated were
processed immediately instead of being queued. This happened because
isCompacting only checked compaction abort controllers, not the branch
summary abort controller.
Include _branchSummaryAbortController in the isCompacting getter so all
existing guards (message queueing, reload blocking) also apply during
branch summarization.
- Add test/utilities.ts with shared helpers (API_KEY, userMsg, assistantMsg, createTestSession)
- Add agent-session-tree-navigation.test.ts with e2e tests for tree navigation
- Add getChildren() method to SessionManager
- Add summaryEntry to navigateTree return type
- Update existing tests to use shared utilities
