Add a keybinding to open the model picker

The model picker is reachable only via the /model slash command (or Enter during
the onboarding ModelSelect phase). There is no keybinding to open it, unlike the
session picker (Alt+R / cmd+B).

Ctrl+Tab / Ctrl+Shift+Tab cycle models without opening the picker, which is not the
same as picking. Users want a key that opens the picker.

Why Ctrl+M is a safe choice (verified)

• The TUI unconditionally pushes KeyboardEnhancementFlags::DISAMBIGUATE_ESCAPE_CODES
  at startup (crates/jcode-tui/src/tui/mod.rs:92-96), i.e. the kitty keyboard protocol.
• Alacritty 0.13+ implements this protocol, so Ctrl+M arrives as CSI 109;5u ->
  KeyCode::Char('m') with CONTROL, distinct from KeyCode::Enter. Ctrl+P already
  works this way as the autopoke toggle, proving the path.
• Routing is gated on modifiers.contains(CONTROL) (input.rs:2877), so a plain m
  still types normally.

Proposed implementation

1. Add model_picker_open: String to KeybindingsConfig
   (crates/jcode-config-types/src/lib.rs:868) defaulting to "ctrl+m".
2. Register it in the keybindings defaults (crates/jcode-config-types/src/keybindings.rs,
   jcode_bindings()) with a field path keybindings.model_picker_open, so the
   conflict checker (crates/jcode-setup-hints/src/keymap/conflicts.rs) picks it up.
3. Wire it in handle_control_key (crates/jcode-tui/src/tui/app/input.rs:1791),
   mirroring the Ctrl+P arm:

   KeyCode::Char('m') => { app.open_model_picker(); true }
   

4. Add the hotkey to the registry (crates/jcode-tui/src/tui/app/hotkey_feedback.rs,
   the Ctrl+P entry at line ~345) so it appears in ⌨ help.

Caveat

If the kitty protocol fails to negotiate (e.g. Alacritty < 0.13), Ctrl+M collapses to
Enter and would collide. This is the same risk Ctrl+P already lives with; acceptable
but worth noting.

Start with KeybindingsConfig and jcode_bindings() in crates/jcode-config-types, then compare the Ctrl+P handling in crates/jcode-tui/src/tui/app/input.rs and its hotkey_feedback.rs registry entry. Check the conflict checker path and verify that Ctrl+M opens the model picker, appears in keyboard help, and leaves plain m behavior unchanged.