fix(cli): avoid exit-time history deep clones

[he-yufeng]

Summary

• replace remaining exit-adjacent getHistory() calls in slash commands and ACP snapshot capture with getHistoryShallow()
• keep restoreHistory() defensive cloning unchanged while avoiding a second deep clone during snapshot capture
• update copy, arena, and ACP session tests to exercise the shallow history path

Fixes #4698.

To verify

• npm run test --workspace=packages/cli -- src/ui/commands/copyCommand.test.ts src/ui/commands/arenaCommand.test.ts src/ui/commands/arenaCommand.agentComplete.test.ts src/acp-integration/session/Session.test.ts
• npx eslint packages/cli/src/ui/commands/copyCommand.ts packages/cli/src/ui/commands/summaryCommand.ts packages/cli/src/ui/commands/arenaCommand.ts packages/cli/src/acp-integration/session/Session.ts packages/cli/src/ui/commands/copyCommand.test.ts packages/cli/src/ui/commands/arenaCommand.agentComplete.test.ts packages/cli/src/acp-integration/session/Session.test.ts
• npm run typecheck --workspace=packages/cli
• git diff --check