test(agent-gui): cover input history acceptance criteria

[ZXLyyds100]

Summary

Closes #2320.

This PR adds focused regression coverage for shell-style AgentGUI composer input history navigation:

• Up/Down navigation, multiline caret behavior, palette priority, and IME composition
• Session-scoped history isolation and older-page loading
• Editing a recalled prompt and submitting the edited payload
• Rebuilding input history from canonical Session messages after reload
• Empty and structured attachment input handling
• Documentation of history retention and application-restart behavior

The production input-history flow already exists in the base branch; this change locks the Issue #2320 acceptance criteria with regression tests and documents the existing ownership and persistence boundary.

Verification

• corepack pnpm@10.11.0 --filter @tutti-os/agent-gui test — 332 test files, 2998 tests passed
• corepack pnpm@10.11.0 --filter @tutti-os/agent-gui typecheck — passed
• node tools/scripts/run-check-changed.mjs --base HEAD^ --push-ready — 11/11 lanes passed
• git diff --check — passed
• Electron + tuttid development startup — verified separately; Codex provider reached ready

Checklist

• No Agent lifecycle semantics were changed; no Host conformance scenario is required.
• The change is focused on one concern.
• Documentation was updated for the retention and restart behavior.
• README/CONTRIBUTING language variants were not applicable.
• The lowest meaningful local checks were run.
• Commits are signed off with DCO.