feat(process): capture stdout/stderr and fan out to subscribers

Co-Authored-By: Claude <noreply@anthropic.com>

internal/process/session.go

@@ -1,6 +1,10 @@
 package process
 
-import "os/exec"
+import (
+	"io"
+	"os/exec"
+	"sync"
+)
 
 // Status represents the status of a process for a workspace.
 type Status struct {
@@ -9,9 +13,32 @@ type Status struct {
 	PID         int    `json:"pid,omitempty"`
 }
 
+// ExitInfo holds the exit reason of a process.
+type ExitInfo struct {
+	Code   int
+	Signal string
+}
+
 // Session holds the runtime state of a started OpenCode process.
 type Session struct {
 	WorkspaceID string
 	Root        string
 	Cmd         *exec.Cmd
+	Stdin       io.WriteCloser
+	Subscribers []Subscription
+	Exit        ExitInfo
+	mu          sync.Mutex
+}
+
+// removeSubscription removes sub from s.Subscribers. It is safe to call when
+// the subscription is no longer present.
+func (s *Session) removeSubscription(sub *subscription) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for i, cur := range s.Subscribers {
+		if cur == sub {
+			s.Subscribers = append(s.Subscribers[:i], s.Subscribers[i+1:]...)
+			return
+		}
+	}
 }