feat(desktop): add Gemini CLI as a tier-2 preset harness

Adds Gemini CLI to the built-in preset harness catalog. Gemini exposes
a native ACP mode via 'gemini --acp' — no adapter needed, no vendor CLI
underlying_cli linkage required.

Ships against the existing preset infrastructure (PresetHarness struct
+ discovery/presets.rs module extracted in #3225). Zero refactor
surface — a single 9-line additive entry into PRESET_HARNESSES.

Signed-off-by: JPeetz <[email protected]>

Author: JPeetz

desktop/src-tauri/src/managed_agents/discovery/presets.rs

@@ -188,6 +188,15 @@ pub(super) const PRESET_HARNESSES: &[PresetHarness] = &[
             Gateway's own environment separately.",
         underlying_cli: None,
     },
+    PresetHarness {
+        id: "gemini",
+        label: "Gemini CLI",
+        command: "gemini",
+        args: &["--acp"],
+        install_instructions_url: "https://github.com/google-gemini/gemini-cli/blob/main/docs/cli/acp-mode.md",
+        install_hint: "Buzz talks to Gemini CLI through its native ACP mode (gemini --acp).",
+        underlying_cli: None,
+    },
 ];
 
 /// Return preset definitions for the spawn/readiness registry.