refactor: replace channel-based AsciiTransportServer with io.ReadWriteCloser ServerSession

[Copilot]

Remove channels from the pkg/asciitransport public server interface. The old AsciiTransportServer leaked implementation details via ResizeEvent() <-chan *ResizeEvent and InputEvent() <-chan *InputEvent.

New interface

type ServerSession interface {
    io.Reader      // reads client InputMsg data
    io.Writer      // writes OutputMsg data to client
    io.Closer
    NextResize() (*ResizeMsg, error)
}

Channels remain internal. The session behaves like a standard file/pipe.

Changes

• pkg/asciitransport/server.go — Replace AsciiTransportServer with ServerSession; implement serverSession struct with Read/Write/Close/NextResize
• pkg/asciitransport/event.go — Add type ResizeMsg = ResizeEvent
• pkg/agent/server/plugin_terminalserver.go — Use io.Copy + NextResize() instead of channel reads and ApplyOpts(WithReader, WithWriter)
• third_party/cmd/ameniicsav1/main.go — Same migration
• pkg/asciitransport/server_test.go — Tests for Read, Write, NextResize, Close, and post-Close behavior

Consumer migration

Before:

server := asciitransport.Server(conn)
go func() {
    for {
        re := <-server.ResizeEvent()
        term.Resize(int(re.Width), int(re.Height))
    }
}()
server.ApplyOpts(asciitransport.WithReader(term), asciitransport.WithWriter(term))
<-server.Done()

After:

session := asciitransport.Server(conn)
go func() {
    for {
        re, err := session.NextResize()
        if err != nil { break }
        term.Resize(int(re.Width), int(re.Height))
    }
}()
go io.Copy(term, session)
go io.Copy(session, term)

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