feat(tui): add Emacs keymap profile with runtime switching

- Introduce built‑in Emacs keymap (`keymap_emacs.toml`) alongside existing Vim layout.
- Add `ui.keymap_profile` and `ui.keymap_path` configuration options; persist profile changes via `:keymap` command.
- Expose `KeymapProfile` enum (Vim, Emacs, Custom) and integrate it throughout state, UI rendering, and help overlay.
- Extend command registry with `keymap.set_vim` and `keymap.set_emacs` to allow profile switching.
- Update help overlay, command specs, and README to reflect new keybindings and profile commands.
- Adjust `Keymap::load` to honor preferred profile, custom paths, and fallback logic.

docs/configuration.md

@@ -73,6 +73,12 @@ These settings customize the look and feel of the terminal interface.
 -   `syntax_highlighting` (boolean, default: `false`)
     Enables lightweight syntax highlighting inside fenced code blocks when the terminal supports 256-color output.
 
+-   `keymap_profile` (string, optional)
+    Set to `"vim"` or `"emacs"` to pick a built-in keymap profile. When omitted the default Vim bindings are used. Runtime changes triggered via `:keymap ...` are persisted by updating this field.
+
+-   `keymap_path` (string, optional)
+    Absolute path to a custom keymap definition. When present it overrides `keymap_profile`. See `crates/owlen-tui/keymap.toml` or `crates/owlen-tui/keymap_emacs.toml` for the expected TOML structure.
+
 ## Storage Settings (`[storage]`)
 
 These settings control how conversations are saved and loaded.