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

[JPeetz]

Summary

• Adds Gemini CLI (gemini --acp, native ACP support, no adapter) as a tier-2 PresetHarness entry — matches the shape of opencode/kimi/cursor.
• Splits the tier-2 preset block (struct, PRESET_HARNESSES, and its two accessor fns) out of discovery.rs into a new sibling module discovery/presets.rs. discovery.rs was already past the desktop file-size ratchet, so a bare addition would have failed just desktop-check. No behavior change from the split itself.
• Adds an inline SVG mark (simple-icons, CC0) and settings catalog copy for Gemini CLI.

Test plan

• cargo fmt --check clean
• cargo clippy --all-targets -- -D warnings clean
• node desktop/scripts/check-file-sizes.mjs passes (discovery.rs down to 1648 lines, new presets.rs at 234)
• cargo test --manifest-path desktop/src-tauri/Cargo.toml discovery — 172 passed
• pnpm exec biome check clean on modified frontend files
• node desktop/scripts/check-px-text.mjs clean
• Live ACP handshake spike against gemini --acp — spawned the real CLI (gemini-cli@0.53.0) locally, sent a JSON-RPC initialize request over stdio matching the exact params buzz-acp sends, got back a well-formed response: agent negotiated down to protocolVersion: 1, returned agentInfo (gemini-cli v0.53.0), agentCapabilities (loadSession, image/audio/embeddedContext prompts, MCP http/sse), and authMethods including gemini-api-key. Confirms command: "gemini", args: ["--acp"] is correct and the harness starts and speaks ACP as configured. (Auth/prompt flow beyond initialize not exercised — out of scope for this spike.)

[JPeetz]

Force-pushed a rewrite. Original branch was 220-line refactor sitting on top of an outdated main — it was extracting the PresetHarness struct and presets.rs module out of discovery.rs. That extraction has already landed upstream in #3225 (Devin preset).

Rewrote the branch as a minimal additive commit against current main: a single 9-line PresetHarness entry appended to PRESET_HARNESSES in desktop/src-tauri/src/managed_agents/discovery/presets.rs. Zero refactor surface. mergeable_state is now blocked (waiting on review, not on conflicts). CI green (Semgrep OSS, zizmor, DCO all pass).

The functional change is unchanged: adds this harness to the built-in tier-2 preset catalog. It uses the same ACP-mode invocation pattern the original PR proposed.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

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[JPeetz]

Rebased on latest main (was 279 commits behind). Same situation as #3519 — old CI failure from Aug 4 with pruned logs. A fresh CI run after approving the workflow would be great. Thanks!