Option to choose the terminal app — open a session's workspace in an external terminal like Ghostty

### Before filing

- [x] I searched open and closed issues for duplicates.
- [x] I am using a recent release and this doesn't already exist.
- [x] This is one request, not several bundled together.

### Closest existing issue

#134 is about the built-in terminal's discoverability, not about choosing which terminal is used — none found for this specifically.

### Is this new, or an improvement?

New capability — Berd can't do this at all today

### The problem, in your terms

Berd sessions come with a built-in dockable terminal in the right rail, and it's always Berd's own terminal. My daily driver is Ghostty, with my own fonts, keybindings, scrollback behavior and shell setup. When I want to poke around a session's attached workspace, I end up wanting my real terminal there, not the embedded one — but there's no way to point Berd at it. As far as I can tell there's no setting to pick a terminal application, and no "open in external terminal" action on the workspace.

### What you do today

Manually open Ghostty myself and `cd` to the workspace path (or ask the chat agent to run `open -a Ghostty `). It works, but it's manual every time and completely disconnected from the workspace/terminal affordances Berd already has.

### What you'd like to see

A setting like "Terminal application" (default: built-in), letting me pick an installed terminal — Ghostty, iTerm2, kitty, WezTerm, Terminal.app — so that the terminal affordances open my chosen app at the workspace path. If a full preference is too much, even a lightweight "Open in external terminal" action next to the built-in terminal (using the chosen or default app) would cover most of it.

### Why this belongs in Berd itself

The terminal button and right-rail docking are app chrome — a skill or agent can run `open -a Ghostty` on request, but it can't change what Berd's own terminal affordance does, add an app-level setting, or make the choice persistent across sessions. Only the app can wire its terminal entry points to a user-chosen application.

### Non-goals

- Not asking to remove or replace the built-in terminal — it should stay the default.
- Not asking to embed Ghostty (or any external emulator) inside the Berd window.
- Not asking for terminal-emulation feature parity in the built-in terminal; launching my external terminal at the right path is enough.

### Alternatives you considered

- Asking the chat agent to launch Ghostty at the workspace path each time — works, but manual and invisible to Berd's UI.
- Living with the built-in terminal — usable, but it will never match a dedicated emulator's configuration, and that's fine; that's what the external option is for.

### Mockups, prior art, or other context

Prior art: VS Code's `terminal.external.osxExec` + "Open in External Terminal" command; JetBrains IDEs' configurable terminal; GitHub Desktop's choice of shell/editor. macOS has no OS-level "default terminal" concept, which is exactly why apps offer this setting themselves.