fix: preserve Claude ACP background task progress

[tiagoefreitas]

Summary

This fixes the stable Claude ACP failure mode where background task activity disappears from the ACP stream and downstream persistence goes stale.

What changed

• patch installed Claude ACP npm packages after install so they opt into CLAUDE_CODE_EMIT_SESSION_STATE_EVENTS=1 when needed
• forward Claude background task system events as ACP session/update notifications instead of dropping them
• move the fallback Claude ACP pin from 0.20.0 to 0.23.1
• add installer regression coverage around the Claude adapter patch path

Why

Claude emits background-task system events such as task_started, task_notification, task_progress, hook_progress, and files_persisted, but the published ACP adapter drops them. When that happens, sandbox-agent persistence stops advancing even though Claude is still actively working.

This patch keeps the ACP stream alive during that background-task window and preserves normal end-of-turn completion through the idle/session-state path.

Closes #291.

Validation

• validated the patch algorithm against published Claude ACP adapter builds 0.22.1, 0.23.1, and 0.26.0
• upgraded the local managed Claude ACP install on this machine to 0.23.1 and confirmed the patched acp-agent.js contains the idle-state and background_event forwarding hooks
• cargo is not available in this environment, so I could not run the Rust crate tests here