Add per-tab prompt input history (#478)

* Add per-tab prompt input history

Keep a bounded in-memory history for each agent tab and route arrow keys according to whether the input box or completed-turn list has focus.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 2a97ff94-dcc3-4410-9fba-72ed719e835f

* Document alternate-scroll state restoration

Explain why the agent pane restores Windows Terminal mode 1007 explicitly instead of using unsupported xterm private-mode save and restore sequences.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 2a97ff94-dcc3-4410-9fba-72ed719e835f

* Restore terminal state on TUI setup failure

Use an armed cleanup guard so failed setup and teardown paths restore raw mode, alternate-screen state, cursor shape, and cursor visibility.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 5c0a29a6-f8d2-452f-8c71-04d22df87777

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Copilot-Session: 2a97ff94-dcc3-4410-9fba-72ed719e835f
Copilot-Session: 5c0a29a6-f8d2-452f-8c71-04d22df87777

Author: vanzue