Bind Cmd+T to new chat, matching the new-tab convention

[shkumbinhasani]

Problem

Cmd+T (Ctrl+T) is unbound in T3 Code. Every tabbed desktop app on macOS opens a new tab with it: Safari, Chrome, VS Code, iTerm, Finder, Terminal. In T3 Code the closest action is "new chat", which is bound to Cmd+N and Cmd+Shift+O, and Cmd+Shift+N for a local chat.

Threads already act like tabs: they sit in a list, Cmd+1..9 jumps between them, Cmd+Shift+[ and ] cycle them. Reaching for Cmd+T to start a new one and getting nothing breaks that model.

Proposal

Add { key: "mod+t", command: "chat.new", when: "!terminalFocus" } to DEFAULT_KEYBINDINGS in packages/shared/src/keybindings.ts, alongside the existing Cmd+N binding rather than replacing it. Cmd+T is free today, so this does not collide with anything.

Related

Cmd+W has the mirror problem: in a chat with no panel tab focused it closes the whole window instead of the thread. Raised as a follow-up on #6791.