AgentSession.switchSession() eagerly called buildDisplaySessionContext()
before setSessionFile, walking the previous session's branch and expanding
every compaction entry's snapcompact archive and openaiRemoteCompaction
replacementHistory into messages. For huge pre-fix sessions that materialized
GBs of data and OOMed in-TUI /resume even after the streaming loader fix.
The snapshot is only needed for same-session reloads, where
#didSessionMessagesChange compares the pre/post message arrays to detect
rollback edits. Different-session switches skip the call entirely; the
error-recovery path rebuilds the previous context on demand from the
restored state so MCP-selection restoration still has its inputs.
Added a regression test (test/agent-session-switch-prev-context.test.ts)
that spies on sessionManager.buildSessionContext across switchSession and
asserts the expected call count and target file per branch.
Fixes#3846
roboomp