Add a bindable shortcut to cite selected assistant text #8

Citing selected assistant text should be reachable through a keyboard command.

Scope and acceptance:

• Expose a configurable cite-selection command in Settings > Keybindings.
• With assistant text selected, the command opens the existing citation/comment flow.
• Without a valid selection, leave the key available to its normal handler.
• Preserve text-field, terminal, browser, and IME input.

Review these upstream candidates when work starts:

• feat(web): add a keybinding command for citing selected assistant text pingdotgg/t3code#34656 adds chat.citeSelection with no default binding.
• feat(web): press R to cite selected assistant text pingdotgg/t3code#25570 proposes a fixed R shortcut.
• Cite is slow to appear after selecting text and has no keyboard shortcut pingdotgg/t3code#27666 reports the missing shortcut.

These were unmerged during the initial research. A shortcut in a PR is not available in the installed app. Prefer a bindable command; decide a default during implementation.