Background subagent parameter hidden from tool schema when experimental flag is enabled

Description

When OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=true, the background parameter on the task tool is completely invisible to agents. The parameter exists in the code but never appears in the tool schema, so agents don't know it's available and never use it.

Root Cause

In packages/opencode/src/tool/task.ts:366:

jsonSchema: flags.experimentalBackgroundSubagents
  ? undefined
  : ToolJsonSchema.fromSchema(BaseParameters),

When the flag is enabled, jsonSchema is set to undefined. The parameter definition itself (in Parameters) includes background, but since the schema is undefined, the agent never sees it.

Expected Behavior

When the experimental flag is on, the background parameter should be visible in the tool schema so agents can discover and use it.

Environment

• OpenCode version: 1.18.17+
• OS: Windows 11
• Config: OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=true