session: define Core interface at consumer, use concrete *session.Session

Core interface defined in 9p/session/core.go lists the methods
the 9P layer needs. The real *session.Session satisfies it implicitly.
Test stubs continue to work via interface satisfaction.

main_test.go

@@ -165,7 +165,7 @@ func newTestSessionManagerWithCore(t *testing.T) *session.Manager {
 		Sink:     sink,
 		ReadFile: func(string) ([]byte, error) { return []byte("#!/bin/sh\n"), nil },
 		MkdirAll: func(string, os.FileMode) error { return nil },
-		NewCore: func(sessionID, agentName, cwd string) (agent.Session, error) {
+		NewCore: func(sessionID, agentName, cwd string) (session.Core, error) {
 			return &stubCore{state: "idle", backend_: "stub", model: "m", agentName: agentName, cwd: cwd}, nil
 		},
 	})
@@ -1221,7 +1221,7 @@ func TestSessionManagerCreateSessionEnvExpansion(t *testing.T) {
 		Sink:     sink,
 		ReadFile: func(string) ([]byte, error) { return []byte("#!/bin/sh\n"), nil },
 		MkdirAll: func(string, os.FileMode) error { return nil },
-		NewCore: func(sessionID, agentName, cwd string) (agent.Session, error) {
+		NewCore: func(sessionID, agentName, cwd string) (session.Core, error) {
 			gotCwd = cwd
 			return &stubCore{state: "idle", backend_: "stub", model: "m", agentName: agentName, cwd: cwd}, nil
 		},
@@ -1248,7 +1248,7 @@ func TestSessionManagerCreateSessionTildeExpansion(t *testing.T) {
 		Sink:     sink,
 		ReadFile: func(string) ([]byte, error) { return []byte("#!/bin/sh\n"), nil },
 		MkdirAll: func(string, os.FileMode) error { return nil },
-		NewCore: func(sessionID, agentName, cwd string) (agent.Session, error) {
+		NewCore: func(sessionID, agentName, cwd string) (session.Core, error) {
 			gotCwd = cwd
 			return &stubCore{state: "idle", backend_: "stub", model: "m", agentName: agentName, cwd: cwd}, nil
 		},

session/core.go

@@ -0,0 +1,55 @@
+package session
+
+import (
+	"context"
+
+	"github.com/simonfxr/pubsub"
+	agent "ollie/session"
+	"ollie/backend"
+)
+
+// Core is the interface that a session implementation must satisfy.
+// Defined here (at the consumer) rather than in ollie/session so that
+// the core package exports only a concrete struct with no interface overhead.
+// Tests can substitute a lightweight stub.
+type Core interface {
+	Submit(ctx context.Context, input string)
+	Interrupt(cause error) bool
+	Inject(prompt string)
+	Queue(prompt string)
+	PopQueue() (string, bool)
+	Bus() *pubsub.Bus
+	IsRunning() bool
+	State() string
+	Reply() string
+	AgentName() string
+	BackendName() string
+	ModelName() string
+	CWD() string
+	SetCWD(dir string) error
+	SetSessionID(newID string) error
+	Context() []backend.Message
+	SystemPrompt() string
+	GenerationParams() backend.GenerationParams
+	SetGenerationParams(params backend.GenerationParams) error
+	CompactionModel() string
+	SetCompactionModel(model string)
+	Usage() string
+	Cost() string
+	CtxSz() string
+	ListModels() string
+	WaitChange(ctx context.Context, field, current string) (string, bool)
+	SaveSession(path string) error
+	Close()
+	Detach() bool
+	ListDetached() []agent.DetachedInfo
+	SignalDetached(pid, signal int) error
+	GetDetachedOutput(pid int) (string, error)
+	DismissDetached(pid int) bool
+	InjectSystemEvent(content string)
+	React(emoji string)
+	ReactTo(responseID, emoji string) error
+	Reactions() map[string]string
+	ToolCallCount() int64
+	SetEnv(key, value string)
+}

session/session.go

@@ -29,7 +29,7 @@ type Session struct {
 	mu         sync.RWMutex
 	id         string
 	uname      string // immutable user principal (numeric UID), set at creation
-	Core       agent.Session
+	Core       Core
 	Ctx        context.Context
 	cancel     context.CancelFunc
 	log        []byte
@@ -46,7 +46,7 @@ type Session struct {
 	modelsCacheAt time.Time
 }
 
-func NewSession(id string, core agent.Session, ctx context.Context, cancel context.CancelFunc) *Session {
+func NewSession(id string, core Core, ctx context.Context, cancel context.CancelFunc) *Session {
 	sess := &Session{id: id, Core: core, Ctx: ctx, cancel: cancel}
 	sess.startEventLog()
 	return sess
@@ -236,7 +236,7 @@ type ManagerConfig struct {
 	MkdirAll    func(string, os.FileMode) error
 	// NewCore, if non-nil, replaces the default backend.New + agent.New
 	// path. It receives the session ID, agent name, and cwd, and returns a Core.
-	NewCore func(sessionID, agentName, cwd string) (agent.Session, error)
+	NewCore func(sessionID, agentName, cwd string) (Core, error)
 	// Strict rejects inline code steps; only tool steps are allowed.
 	Strict bool
 	// Yolo skips the landrun sandbox.
@@ -1122,7 +1122,7 @@ func (s *Manager) CreateSession(args []string) (string, error) {
 		return "", fmt.Errorf("session already exists: %s", sessID)
 	}
 
-	var core agent.Session
+	var core Core
 	var sessPtr *Session
 	uname := s.nextUname()
 	if s.cfg.NewCore != nil {