Option to run long bash commands in the background instead of blocking in the foreground #365

Description

Add a config option, for example bash.prefer_background_for_long: true (default false, so nothing changes for existing users). When it's enabled in a primary (non-subagent) session:

• Quick commands behave exactly as they do today and return inline.
• The bash tool description recommends background: true for commands known to be long, such as builds, full test suites, deploys and installs. The agent then ends its turn or does other work, and the completion reminder wakes it.
• wait: true is presented as the exception, for when the result is needed right away. It's no longer the recommended way to run long commands.
• Subagents keep the current guidance, since they need results before they report back. subagent_background already covers them.

Today the description says the opposite: "use bash({ command, wait: true }) when you know it is long", "background: true is ONLY for when you have OTHER useful work", and "Do NOT background a command and then immediately bash_watch it".

Use case

In an interactive session, the main agent is often the user's point of contact while subagents do the work. When it blocks on a multi-minute build, test run or deploy, it isn't available to the user until the command finishes or the user sends a message to detach it. We'd rather it start the job in the background, stay free, and pick up the result when the completion reminder arrives.

A prompt can't fix this. Our agent prompt says "Run long-running commands in the background", but the tool description says the reverse, and the model usually follows the tool description. So the agent gets contradictory instructions and doesn't behave consistently.

This is a preference, not a bug. #124 asked for the opposite, keeping long validation commands in the foreground so the agent doesn't burn context while waiting, and that was solved with a config switch. An opt-in option would let both kinds of users get what they want.

Environment: @cortexkit/aft-opencode 0.57.2, OpenCode Desktop on macOS.