Add a keyboard shortcut for the Pull Requests page

[jpsirois]

The Pull Requests page has no keyboard shortcut. You can reach it from the sidebar or the command palette ("Open pull requests"), but usage.open (mod+u) shows the pattern for opening a global page with one key.

Proposal: add a pullRequests.open keybinding command.

• Default mod+shift+p, when: "!terminalFocus". No other default binding or desktop menu accelerator uses it.
• Shown as the shortcut on the existing "Open pull requests" palette action.
• Does nothing when no connected environment supports pull requests, same as the palette action.
• Rebindable in Settings → Keybindings as "Pull Requests: Open".

Scope is small: one command in packages/contracts, one default in packages/shared, a handler next to usage.open in CommandPalette.tsx, and a short section in docs/user/keybindings.md. Mobile has no keyboard shortcuts, so it is unaffected.

Open question: is mod+shift+p the right default, or would you prefer another key or no default?

I have a working branch and can open a PR if you approve the direction.