feat(acp): support native session forks

[danielkov]

Summary

Adds native ACP session/fork routing for harnesses that advertise the capability while retaining a safe transcript-cloning fallback. Fork copies discard session-bound provider state without modifying the source transcript.

Motivation

Fallback forks could copy OpenAI continuation metadata bound to the source session, causing Responses continuation metadata binding is invalid. Kit also did not expose the native fork operation already available in the ACP protocol.

Impact

Kit-backed ACP subagent forks can share one ACP process while preserving independent durable session identities and inherited model settings. Cancelling or timing out a fork leaves the source and sibling sessions usable, and branches rebuild provider continuation state on their next response.

Technical details

Fork-safe transcripts

Fork copies remove openai.responses.continuation.v1, openai.subscription.v1, and generated assistant media. The source transcript remains untouched.

Session lifecycle

Native forks are claimed and activated as durable sessions, inherit source model and reasoning-effort selection, and carry parent context for nested subagent reporting. Late fork responses are closed instead of orphaned, and per-session close cleans up descendants without terminating the shared ACP process.

Compatibility

Harnesses without native fork support continue to use sanitized transcript cloning.

[kit-code-agent]

Found two lifecycle issues in the native fork path: failed attachment can leave an unreachable durable session, and failed close can leave descendant state visible after its parent is removed.

[kit-code-agent]

The two prior lifecycle findings are addressed. One new limited roster-lifecycle issue remains in the descendant tombstone handling.

[kit-code-agent]

No issues found. The changes look good to merge.