Add experimental background agents

[nexxeln]

summary

• add task(background=true) and task_status behind OPENCODE_EXPERIMENTAL_BACKGROUND_AGENTS
• run background subagents through the background job service with parent result injection
• cancel background agents on parent/child session deletion and parent interruption
• label background task cards in TUI/web UI

testing

• bun typecheck from packages/opencode
• bun test test/tool/task.test.ts test/tool/task_status.test.ts test/tool/registry.test.ts test/tool/parameters.test.ts from packages/opencode
• pre-push bun turbo typecheck