build: make GPUI an optional feature to allow TUI-only builds

[yamaceay]

Problem

The GPUI desktop UI compiles Metal shaders at build time via a Cargo
build-script. This requires Xcode developer tools (xcrun must locate the
metal binary). On machines without Xcode, every cargo build failed —
including builds of the terminal TUI, which needs no Metal at all:

error: gpui_macos@0.1.0: metal shader compilation failed:
xcrun: error: unable to find utility "metal", not a developer tool or in PATH

The root cause was twofold:

1. A transitive dependency chain pulling in gpui_macos unconditionally:

   code-assistant → terminal (crate) → gpui → gpui_macos (Metal build-script)
   code-assistant → terminal_view   → gpui → gpui_macos
   code-assistant → gpui, gpui_platform, gpui-component → gpui_macos
   

2. terminal, terminal_test_app, and terminal_view were listed as unconditional workspace members, so Cargo always compiled them regardless of feature flags — even with --no-default-features.

---

Solution

Two complementary fixes:

1. Introduce a gpui-ui Cargo feature that makes all GPUI-related code opt-in.
2. Remove terminal, terminal_test_app, and terminal_view from the workspace members list so they are only compiled when pulled in transitively via the gpui-ui feature.

Build command	Result
cargo build --no-default-features	TUI-only binary — no Xcode required ✓
cargo build --features gpui-ui	Full binary including desktop UI (requires Xcode)
cargo build	Same as --no-default-features (default unchanged)

This PR is the foundation for the autocomplete PR series (#131–#135), which
all depend on TUI-only builds working.

---

Changes

Cargo.toml (workspace)

• Removed terminal, terminal_test_app, terminal_view from workspace members.
  They are still available as path dependencies when gpui-ui is active.

crates/code_assistant/Cargo.toml

• gpui, gpui_platform, gpui-component, terminal, terminal_view
  all made optional = true.
• New feature: gpui-ui = ["dep:gpui", "dep:gpui_platform", "dep:gpui-component", "dep:terminal", "dep:terminal_view"].
• Removed unused gpui from [dev-dependencies].

src/app/mod.rs, src/ui/mod.rs

• pub mod gpui gated with #[cfg(feature = "gpui-ui")].

src/ui/backend.rs

• GpuiTerminalCommandExecutor import gated.
• Usage site falls back to DefaultCommandExecutor when feature is off.

src/ui/ui_events.rs

• terminal::StyledLine re-exported under #[cfg(feature = "gpui-ui")].
• Zero-sized stub struct StyledLine; defined under #[cfg(not(feature = "gpui-ui"))].

src/main.rs

• --ui logging, model resolution, and app::gpui::run() all gated.
• Passing --ui without the feature prints a clear error.

src/cli.rs

• UiSettings::load() gated; TUI-only builds fall through to alphabetical-first-model fallback.

---

Test plan

• cargo build --no-default-features completes without Xcode/Metal tools.
• ./target/debug/code-assistant launches the terminal TUI normally.
• ./target/debug/code-assistant --version prints the version string.
• ./target/debug/code-assistant --ui prints the feature-missing error message.
• (On a machine with Xcode) cargo build --features gpui-ui succeeds and
  --ui launches the GPUI desktop interface.

[yamaceay]

Potentially in conflict with #130, need to align first

[stippi]

Solved differently/again on main after the big crate restructuring.