feat(shell): add parallel bash shell subsystem auto-started per workspace

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

internal/shell/session.go

@@ -0,0 +1,39 @@
+package shell
+
+import (
+	"io"
+	"os/exec"
+	"sync"
+)
+
+// Status represents the status of a shell for a workspace.
+type Status struct {
+	WorkspaceID string `json:"workspaceId"`
+	Running     bool   `json:"running"`
+	PID         int    `json:"pid,omitempty"`
+}
+
+// ExitInfo holds the exit reason of a shell.
+type ExitInfo struct {
+	Code   int
+	Signal string
+}
+
+// Session holds the runtime state of a started shell.
+type Session struct {
+	WorkspaceID string
+	Root        string
+	Cmd         *exec.Cmd
+	Stdin       io.WriteCloser
+	Subscribers map[Subscription]struct{}
+	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()
+	delete(s.Subscribers, sub)
+}