Add --json support to gh agent-task list

Add --json, --jq, and --template flags to `gh agent-task list`,
consistent with the pattern used by `gh pr list --json`,
`gh issue list --json`, etc.

This implements the ExportData interface on capi.Session and defines
SessionFields for the available JSON fields:
id, name, status, repository, createdAt, updatedAt,
pullRequestNumber, pullRequestUrl.

Closes https://github.com/cli/cli/issues/12805 (partial)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

pkg/cmd/agent-task/capi/sessions.go

@@ -102,6 +102,58 @@ type SessionError struct {
 	Message string
 }
 
+// SessionFields defines the available fields for JSON export of a Session.
+var SessionFields = []string{
+	"id",
+	"name",
+	"status",
+	"repository",
+	"createdAt",
+	"updatedAt",
+	"pullRequestNumber",
+	"pullRequestUrl",
+}
+
+// ExportData implements the exportable interface for JSON output.
+func (s *Session) ExportData(fields []string) map[string]interface{} {
+	data := make(map[string]interface{}, len(fields))
+	for _, f := range fields {
+		switch f {
+		case "id":
+			data[f] = s.ID
+		case "name":
+			data[f] = s.Name
+		case "status":
+			data[f] = s.State
+		case "repository":
+			if s.PullRequest != nil && s.PullRequest.Repository != nil {
+				data[f] = s.PullRequest.Repository.NameWithOwner
+			} else {
+				data[f] = nil
+			}
+		case "createdAt":
+			data[f] = s.CreatedAt
+		case "updatedAt":
+			data[f] = s.LastUpdatedAt
+		case "pullRequestNumber":
+			if s.PullRequest != nil {
+				data[f] = s.PullRequest.Number
+			} else {
+				data[f] = nil
+			}
+		case "pullRequestUrl":
+			if s.PullRequest != nil {
+				data[f] = s.PullRequest.URL
+			} else {
+				data[f] = nil
+			}
+		default:
+			data[f] = nil
+		}
+	}
+	return data
+}
+
 type resource struct {
 	ID                   string            `json:"id"`
 	UserID               uint64            `json:"user_id"`

pkg/cmd/agent-task/list/list.go

@@ -25,6 +25,7 @@ type ListOptions struct {
 	CapiClient func() (capi.CapiClient, error)
 	Web        bool
 	Browser    browser.Browser
+	Exporter   cmdutil.Exporter
 }
 
 // NewCmdList creates the list command
@@ -54,6 +55,8 @@ func NewCmdList(f *cmdutil.Factory, runF func(*ListOptions) error) *cobra.Comman
 	cmd.Flags().IntVarP(&opts.Limit, "limit", "L", defaultLimit, "Maximum number of agent tasks to fetch")
 	cmd.Flags().BoolVarP(&opts.Web, "web", "w", false, "Open agent tasks in the browser")
 
+	cmdutil.AddJSONFlags(cmd, &opts.Exporter, capi.SessionFields)
+
 	return cmd
 }
 
@@ -91,6 +94,10 @@ func listRun(opts *ListOptions) error {
 		return cmdutil.NewNoResultsError("no agent tasks found")
 	}
 
+	if opts.Exporter != nil {
+		return opts.Exporter.Write(opts.IO, sessions)
+	}
+
 	if err := opts.IO.StartPager(); err == nil {
 		defer opts.IO.StopPager()
 	} else {

pkg/cmd/agent-task/list/list_test.go

@@ -99,6 +99,7 @@ func Test_listRun(t *testing.T) {
 		capiStubs      func(*testing.T, *capi.CapiClientMock)
 		limit          int
 		web            bool
+		jsonFields     []string
 		wantOut        string
 		wantErr        error
 		wantStderr     string
@@ -286,6 +287,33 @@ func Test_listRun(t *testing.T) {
 			wantStderr:     "Opening https://github.com/copilot/agents in your browser.\n",
 			wantBrowserURL: "https://github.com/copilot/agents",
 		},
+		{
+			name: "json output",
+			tty:  false,
+			capiStubs: func(t *testing.T, m *capi.CapiClientMock) {
+				m.ListLatestSessionsForViewerFunc = func(ctx context.Context, limit int) ([]*capi.Session, error) {
+					return []*capi.Session{
+						{
+							ID:            "abc-123",
+							Name:          "s1",
+							State:         "completed",
+							CreatedAt:     sampleDate,
+							LastUpdatedAt: sampleDate,
+							ResourceType:  "pull",
+							PullRequest: &api.PullRequest{
+								Number: 101,
+								URL:    "https://github.com/OWNER/REPO/pull/101",
+								Repository: &api.PRRepository{
+									NameWithOwner: "OWNER/REPO",
+								},
+							},
+						},
+					}, nil
+				}
+			},
+			jsonFields: []string{"id", "name", "status", "repository", "pullRequestNumber", "pullRequestUrl"},
+			wantOut:    "[{\"id\":\"abc-123\",\"name\":\"s1\",\"pullRequestNumber\":101,\"pullRequestUrl\":\"https://github.com/OWNER/REPO/pull/101\",\"repository\":\"OWNER/REPO\",\"status\":\"completed\"}]\n",
+		},
 	}
 
 	for _, tt := range tests {
@@ -316,6 +344,12 @@ func Test_listRun(t *testing.T) {
 				},
 			}
 
+			if tt.jsonFields != nil {
+				exporter := cmdutil.NewJSONExporter()
+				exporter.SetFields(tt.jsonFields)
+				opts.Exporter = exporter
+			}
+
 			err := listRun(opts)
 			if tt.wantErr != nil {
 				assert.Error(t, err)