buzz-acp: OpenCode-backed agents hang/timeout — session/set_config_option sends Claude-only permission mode values OpenCode rejects

## Summary

`buzz-acp` agents backed by `agent_command_override: "opencode"` reliably hang and time out during initialization ("Request timeout — agent did not respond within 60s") on every model, on Linux. Root cause: `--permission-mode` sends a `session/set_config_option` call with a value from buzz-acp's hardcoded Claude-flavored enum (`default`/`accept-edits`/`bypass-permissions`/`dont-ask`/`plan`), but OpenCode's ACP `mode` config option only accepts `build`/`plan`. OpenCode correctly returns a JSON-RPC error for the unrecognized value, but buzz-acp appears to hang waiting rather than handling the error (it only gracefully falls back for `"Method not found"`, e.g. from `claude-code-acp`, which doesn't implement `session/set_config_option` at all).

## Reproduction (direct ACP round-trip against `opencode acp`, no buzz involved)

```
opencode --version # 1.18.9
```

```jsonc
// 1. initialize -> ok
{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":1,"clientCapabilities":{}}}

// 2. session/new -> ok, returns configOptions with a "mode" option whose only
// valid values are "build" and "plan"
{"jsonrpc":"2.0","id":2,"method":"session/new","params":{"cwd":"/home/me","mcpServers":[]}}

// 3. session/set_config_option with buzz-acp's default value -> hard error
{"jsonrpc":"2.0","id":3,"method":"session/set_config_option","params":{"sessionId":"","configId":"mode","value":"bypassPermissions"}}
```

Response to (3):
```json
{"jsonrpc":"2.0","id":3,"error":{"code":-32602,"message":"Invalid params: mode not found: bypassPermissions","data":{"mode":"bypassPermissions"}}}
```

The same happens for every other `PermissionMode` variant buzz-acp can send (`default`, `accept-edits`, `dont-ask`) since none map to OpenCode's `build`/`plan` vocabulary — confirmed by inspecting `session/new`'s returned `configOptions` for the `mode` category. There is currently **no `BUZZ_ACP_PERMISSION_MODE` value that works** with an OpenCode-backed agent.

## Observed in buzz-desktop logs

```
buzz-acp starting: ... agent_cmd=/home/user/.bun/bin/opencode ... permission_mode=bypassPermissions ...
ERROR buzz_acp: agent timed out during init (60s) agent=0
ERROR buzz_acp: agent initialize failed: Request timeout — agent did not respond within 60s agent=1
ERROR buzz_acp: agent initialize failed: Request timeout — agent did not respond within 60s agent=2
```

No corresponding opencode log file is ever created for the failed instances (`~/.local/share/opencode/log/`), and the buzz session logger's per-session `.md` file stays stuck at "Status: Active" with zero further content — consistent with the agent process being stuck waiting on the `session/set_config_option` response rather than crashing.

This reproduces on every model (`spark/glm-5.2`, `spark/qwen36-35b-a3b`) and is unrelated to auth/network — a bare `session/new` + `session/prompt` round trip against the same `opencode acp` process (skipping the `session/set_config_option` call) completes in a few seconds.

## Environment

- Buzz Desktop 0.5.2, Linux x86_64
- OpenCode 1.18.9
- `agent_command_override: "opencode"` on a managed agent (per `~/.local/share/xyz.block.buzz.app/agents/managed-agents.json`)

## Suggested fix

Either:
1. Treat any non-"Method not found" error from `session/set_config_option` as non-fatal (log and continue without the mode override), the same way "Method not found" is already handled, or
2. Query `session/new`'s returned `configOptions` for the `mode` category's valid `options` and only attempt `session/set_config_option` if the configured `PermissionMode`'s wire value is actually in that list, or
3. Add an adapter-specific permission-mode mapping (similar in spirit to #2884's request for a Claude `auto` variant) so OpenCode's `build`/`plan` modes can be targeted directly instead of assuming Claude's vocabulary.

Related but distinct: #3660 reports the same overall symptom ("Opencode acp not working" / 60s init timeout) but attributes it on Windows to a missing `acp` subcommand argument. On Linux, `buzz-acp`'s default `--agent-args acp` is applied correctly (confirmed the spawned command is `opencode acp`), so that isn't the cause here — this is a distinct bug in the `session/set_config_option` error-handling path.