Add `global.copy` to the TUI keymap schema, runtime keymap,
and interactive remap action list so the Copy shortcut can be
remapped or unbound like other global shortcuts.
Move the existing `ctrl-o` default into a new `v3` preset while
preserving older presets, and update docs plus coverage for the
remapped copy behavior.
Clarify that shifted letter bindings are matched compatibly when
terminals emit uppercase letters without an explicit SHIFT modifier.
Update config guidance, the default keymap template, and the action
matrix with examples for shift-i, shift-a, and shift-o mapping to I, A,
and O.
Expose Vim bindings through the TUI keymap config and add a startup
toggle so Vim mode can be enabled by default.
Wire runtime keymap handling for Vim actions, add
`tui.vim_mode_default` to config parsing/runtime config, apply the
default in ChatWidget startup paths, and update docs/default
keymap/schema accordingly.
Publish keymap system documentation first so the implementation stack can
be reviewed against explicit behavior and invariants.
This commit adds the keymap system guide, action matrix, default keymap
template, and config/example documentation updates, plus a rollout plan
used to stage the additive refactor and validation work.
