fix(acp): opencode preset missing 'acp' arg causes agent timeout

## Problem

The OpenCode preset harness in `presets.rs` defines `args: &[]`, but the install hint correctly states "Buzz talks to OpenCode through its CLI's ACP mode (opencode acp)".

Without the `acp` argument, opencode starts in TUI mode and never responds to the ACP JSON-RPC handshake, causing buzz-acp to timeout after 60 seconds with "agent initialize failed: Request timeout".

This affects every user who selects OpenCode as their agent runtime.

## Steps to reproduce

1. Install opencode CLI (`npm install -g opencode-ai`)
2. Open Buzz Desktop, complete onboarding
3. Select OpenCode as agent runtime
4. @mention any agent
5. Agent appears "online" but never responds

## Root cause

`desktop/src-tauri/src/managed_agents/discovery/presets.rs` line ~173:

```rust
// Current (broken)
PresetHarness {
id: "opencode",
label: "OpenCode",
command: "opencode",
args: &[], // ← missing "acp"
install_hint: "...through its CLI's ACP mode (opencode acp).",
...
},
```

## Fix

```rust
args: &["acp"],
```

All other presets that need an explicit ACP mode flag already carry it:
- devin: `args: &["acp"]` ✓
- cursor: `args: &["acp"]` ✓
- omp: `args: &["acp"]` ✓
- kimi: `args: &["acp"]` ✓
- openclaw: `args: &["acp"]` ✓
- opencode: `args: &[]` ✗ ← this issue

## Tested

Verified locally by editing `managed-agents.json` to set `agent_args: ["acp"]`. All three default agents (Fizz, Honey, Pollen) initialize successfully and respond to @mentions. Without the fix, all agents timeout consistently.