fix(acp): preserve slash command prompt lifecycle - #49208

Problem

ACP treated every slash command as an immediate action: session/prompt returned end_turn as soon as session.command acknowledged the request, so the model work started by /init, /review, MCP prompt commands, and config template commands was never streamed to the client.

Change

• Commands receive a messageID and return an explicit outcome: { type: "immediate" } (or void) when they finish inside the request, or { type: "prompt", inboxID } when they admitted session input. Core rejects a prompt outcome whose inboxID differs from the invocation messageID, since clients follow that ID.
• Built-in and config commands admit with the invocation ID and return the prompt outcome. Subagent commands report the child admission.
• session.command accepts an optional id and returns the outcome. Client and OpenAPI regenerated.
• ACP passes its turn start ID as the command id, returns immediately only for immediate, and otherwise keeps the prompt pending until the work completes, including child sessions spawned by subagent commands and the parent's follow-up run. Cancelling interrupts open children and any session that starts after the cancel.

Tests

• New subprocess tests with a gated model: template command, /init, immediate plugin commands (no model requests), subagent command, and cancellation during template and subagent work.
• In-process tests for pending-until-complete, child-session outcomes, and cancellation ordering; core tests for outcome validation.
• ACP suite 94 → 103 passing.