New chat shortcut (Ctrl/Cmd+K) conflicts with j/k session navigation

[TomBanksAU]

Feature Description

Change the new conversation keyboard shortcut from Ctrl/Cmd+K to Ctrl/Cmd+Shift+O.

Motivation

Ctrl/Cmd+K now conflicts with the j/k session navigation bindings added in #3845. Users who want to navigate sessions with j and k may accidentally trigger a new chat when pressing Ctrl+k.

Additionally, Ctrl+Shift+O matches the convention used by ChatGPT and Claude for opening a new conversation, making it more intuitive for users coming from other AI chat platforms.

Proposed Solution

Change the key binding so that Ctrl/Cmd+Shift+O triggers newSession() instead of Ctrl/Cmd+K. Update the tooltip on the New Conversation button to show (Cmd+Shift+O).

A PR with the change is at #3953.

Acceptance Criteria

• Ctrl/Cmd+Shift+O creates a new conversation from anywhere
• Tooltip on the New Conversation button reads (Cmd+Shift+O)
• Existing tests pass with updated assertions

Alternatives Considered

• Keep Ctrl+K and change j/k: j/k is a well-established vim-style navigation convention; changing it would be more disruptive
• Ctrl+N: conflicts with browser new-window shortcut
• Ctrl+T: conflicts with browser new-tab shortcut