feat(tui): add configurable Vim keymap and startup default

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.

docs/example-config.md

@@ -5,6 +5,9 @@ For a full sample configuration, see [this documentation](https://developers.ope
 ## Keymap snippet
 
 ```toml
+[tui]
+vim_mode_default = false
+
 [tui.keymap]
 preset = "latest" # currently points to v1 defaults