Make tools widget shortcut configurable

[TxHawks]

This pull request enables users to configure the subagent tools widget shortcut config.json. It:

• Preserves ctrl+j as the default
• Reflects the configured shortcut in widget hints
• Warn and fall back to ctrl+j when the configured shortcut is invalid
• Documents the new keybindings.toggleToolsWidget option in README.md
• Adds tests

[nertzy]

Thanks for adding this - the config knob is the right primitive. One thing worth deciding before merge: the default stays ctrl+j, which collides with pi's built-in tui.input.newLine (bound to shift+enter and ctrl+j). On current pi that means every user gets an Extension shortcut conflict: 'ctrl+j' is built-in warning on subagent launch, and the extension wins, so ctrl+j stops inserting a newline in subagent panes out of the box.

Since it hits the default path, would you be open to moving the default off ctrl+j too (or skipping registration when the resolved shortcut equals a pi built-in)? That'd make it conflict-free by default and keep the knob for anyone who wants ctrl+j. Happy to send a follow-up commit.