feat(agent-gui): add input history navigation

[devRickyyy]

Summary

Add shell-style input history navigation to the Agent conversation composer.

User goal

Let users quickly recall, edit, and resend their previous messages without copying text manually.

Acceptance criteria

• When the composer is empty, pressing ArrowUp inserts the most recent user-sent text for the current conversation.
• Repeated ArrowUp and ArrowDown navigate through the current conversation’s sent-message history.
• Navigating down past the newest history entry restores the empty-composer state.
• A recalled message remains editable and can be sent again.
• When the composer already has content, including multiline text, ArrowUp/ArrowDown preserve normal caret navigation instead of entering history navigation.
• Do not intercept key events while an IME composition is active.
• Conversation switching must not leak history navigation state or text between conversations.
• Empty or non-text-only sends must have defined behavior and must not produce invalid recalled entries.

Implementation notes

• Keep sent-message history distinct from unsent composer drafts.
• Define a bounded retention policy and whether history persists after app restart; document the chosen behavior.
• Preserve the existing send flow and keyboard shortcuts.

Test coverage

Add automated coverage for:

• first / repeated up and down navigation
• returning to the draft/empty state
• editing and resending a recalled entry
• multiline caret behavior
• IME composition
• conversation switching and history isolation

Source

Product feedback: the composer should recall the previous user input when ArrowUp is pressed, with ArrowDown for reverse navigation.