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.
Felipe Coury