[Feature]: Configurable editor keybinding presets (Vim, Emacs, …) for the built-in Monaco editor

**Problem**

When orchestrating a fleet of agents, I still frequently need to make quick manual edits in Orca's built-in editor. Since the editor isn't modal (no Vim mode), it breaks muscle memory and slows down these quick edits.

**Why extensions don't solve this**

Orca embeds the **Monaco editor** (the editor component), not a full VS Code fork, so VS Code extensions like `VSCodeVim` can't be installed. The right approach for Monaco is integrating dedicated keybinding libraries directly.

**Proposal**

Add an **"Editor keybindings" preset** setting (next to the existing font / minimap options), defaulting to **Default (none)**:

- **Vim** — via [`monaco-vim`](https://github.com/brijeshb42/monaco-vim) (mature, includes a status bar for mode display)
- **Emacs** — via [`monaco-emacs`](https://github.com/aioutecism/monaco-emacs)
- Framed as an extensible preset list so other modes (e.g. nano) can be community-contributed later.

**Scope / compatibility**

- Off by default → zero impact on existing users.
- Per-editor-instance lifecycle handled (dispose on unmount, since worktrees spawn multiple editors).
- Ensure interop with autosave (e.g. `:w` shouldn't conflict).

**Offer**

Happy to work on the PR (starting with Vim) if the maintainers are open to this direction. Wanted to align on approach and settings placement first.