feat(desktop): add zcode-acp preset to the harness catalog

[jpalmae]

Following up on #3131 (closed as superseded by the harness catalog shipped in v0.5.0 / #2773), this adds zcode-acp as a tier-2 preset so ZCode is one-click discoverable in the Desktop runtime gallery — the data-entry PR @wpfleger96 suggested in #3131 (comment).

What

zcode-acp is a standalone ACP adapter (mirroring amp-acp / hermes-acp) that drives the ZCode CLI through its stdio app-server. It speaks ACP v1 directly, so no other Buzz-side change is needed beyond this preset entry.

• Adapter repo: https://github.com/jpalmae/zcode-acp (Apache-2.0)
• ACP parity with codex-acp: streaming text + reasoning, tool calls (tool.updated → ToolCall/ToolCallUpdate), session/requestPermission bridged bidirectionally, authenticate/logout (Z.AI OAuth init/poll + credential store compatible with zcode login), UsageUpdate.

Changes (small data-entry, as suggested)

File	Change
desktop/src-tauri/src/managed_agents/discovery/presets.rs	One new PresetHarness entry in PRESET_HARNESSES, shaped exactly like amp: command: \"zcode-acp\", args: &[], underlying_cli: Some(\"zcode\") (so Desktop reports AdapterMissing when the ZCode CLI is present but the adapter is not).
desktop/src/features/onboarding/ui/RuntimeIcon.tsx	Map \"zcode-acp\" → /harness-logos/zcode-acp.svg in PRESET_LOGOS (satisfies presetLogos.test.mjs).
desktop/public/harness-logos/zcode-acp.svg	Original, CC0/public-domain placeholder — a monospace "ZC" wordmark on a rounded square. Not derived from any third-party mark, so there are no trademark/redistribution concerns. Happy to swap in an official Z.ai mark under redistribution terms if/when one is provided.
desktop/public/harness-logos/CREDITS.md	Provenance row for the placeholder.

Why underlying_cli: Some(\"zcode\")

zcode-acp is an adapter that spawns node zcode.cjs ... — i.e. it wraps a separately-installed vendor bundle. Mirroring amp (which sets underlying_cli: Some(\"amp\")), this makes Desktop distinguish “ZCode installed but the adapter missing” from “neither installed”, surfacing the right install guidance.

Verification

• presetLogos.test.mjs assertions replicated locally (same regex the test uses): every PRESET_HARNESSES id has either an inline RUNTIME_MARKS entry or a PRESET_LOGOS entry whose file exists on disk, and there are no orphan PRESET_LOGOS keys. ✓
• The new preset entry is byte-for-byte the same shape as the existing amp preset (same fields, same order, trailing comma), so it compiles under the crate's rustfmt/clippy gates. The full cargo check -p buzz-desktop requires a prebuilt buzz-acp binary in build.rs that isn't available outside the release pipeline, so I could not run it end-to-end locally — but the change is additive data-only and mirrors a sibling.

Out of scope

• The placeholder logo is intentionally neutral. If maintainers prefer a different treatment (e.g. leave it as the generic terminal-glyph fallback by not adding a logo at all and special-casing it like codex), happy to adjust.
• Headless buzz-acp already works without this PR (per feat(buzz-acp): register zcode-acp as a known ACP agent #3131 feedback); this PR is purely about Desktop discoverability.