tui: align keybinds with standard terminal/readline/screen conventions

[gw0]

Aligned the keybinds with standard terminal/readline/screen conventions.

"session_child_cycle": "ctrl+tab", // was "ctrl+right"
"session_child_cycle_reverse": "ctrl+shift+tab", // was "ctrl+left"

• Changing ctrl+left/ctrl+right as it conflicts with standard terminal/readline convention to quickly move the cursor word by word (instead of by char).
• Changed to ctrl+tab/ctrl+shift+tab to make cycling through child agents similar to cycling through primary agents (tab/shift+tab).
• Result: Unfortunately, the cursor is not moving yet as something is interfering with the readline configuration (/etc/inputrc) and also both ctrl+tab/ctrl+shift+tab are picked up as there was no ctrl. :(

"messages_page_up": "ctrl+alt+b,pageup", // was "pageup"
"messages_page_down": "ctrl+alt+f,pagedown", // was "pagedown"

• Adding GNU/screen-like full screen movement keys ctrl+alt+b/ctrl+alt+f to match half screen movement keys (opencode: ctrl+alt+u/ctrl+alt+d, screen: ctrl+u/ctrl+d) (https://www.gnu.org/software/screen/manual/html_node/Movement.html).
• Leaving pageup/pagedown as it matches some old terminals behavior.

"messages_first": "ctrl+g", // was "ctrl+g,home"
"messages_last": "ctrl+alt+g", // was "ctrl+alt+g,end"

• Removing home/end as it conflicts with standard terminal/readline convention of moving the cursor to the beginning/end of the current message.
• Result: Moving to beginning/end of message just works.

"history_previous": "ctrl+up", // was "up"
"history_next": "ctrl+down", // was "down"

• Changing up/down as it conflicts with standard terminal/readline convention to move the cursor up/down across a multi-line message.
• Result: Moving up/down across a multi-line message just works.