Kit ACP server cannot expose native session forks

[danielkov]

Status: unresolved; intentionally out of scope for the Kit-owned ACP server.

Kit's ACP client supports session/fork, including capability detection,
concurrent sibling sessions, and session/close. Kit's ACP server cannot
currently advertise or handle the same method.

Current boundary

Kit owns its ACP server and per-session actor in src/protocols/acp.rs, using
agent-client-protocol for transport and the public AcpIntegration API for
conversion, observer routing, and session binding. The server deliberately
advertises and handles initialize, session/new, session/prompt,
session/cancel, and session/close, but does not advertise or register
session/fork.

src/protocols/acp.rs has a protocol-level test that locks in this boundary:
initialization omits the fork capability and a session/fork request returns
method-not-found. Native server-side fork remains a separate design decision; it
must not be inferred from Kit's client-side support for forking nested ACP
harnesses.