Record agentic invocations in User-Agent header

Detect which AI coding agent is invoking gh by checking well-known
environment variables and include the agent name in the User-Agent
header sent to GitHub APIs.

Supported agents: Codex, Gemini CLI, Copilot CLI, OpenCode,
Claude Code, and Amp. Generic AI_AGENT env var is also supported
with validation to prevent header injection.

Fixes github/cli#1111

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

api/client.go

@@ -15,7 +15,6 @@ import (
 )
 
 const (
-	accept          = "Accept"
 	apiVersion      = "X-GitHub-Api-Version"
 	apiVersionValue = "2022-11-28"
 	authorization   = "Authorization"

api/http_client.go

@@ -18,6 +18,7 @@ type tokenGetter interface {
 
 type HTTPClientOptions struct {
 	AppVersion         string
+	InvokingAgent      string
 	CacheTTL           time.Duration
 	Config             tokenGetter
 	EnableCache        bool
@@ -48,8 +49,13 @@ func NewHTTPClient(opts HTTPClientOptions) (*http.Client, error) {
 		clientOpts.LogVerboseHTTP = opts.LogVerboseHTTP
 	}
 
+	ua := fmt.Sprintf("GitHub CLI %s", opts.AppVersion)
+	if opts.InvokingAgent != "" {
+		ua = fmt.Sprintf("%s Agent/%s", ua, opts.InvokingAgent)
+	}
+
 	headers := map[string]string{
-		userAgent:  fmt.Sprintf("GitHub CLI %s", opts.AppVersion),
+		userAgent:  ua,
 		apiVersion: apiVersionValue,
 	}
 	clientOpts.Headers = headers

api/http_client_test.go

@@ -20,6 +20,7 @@ func TestNewHTTPClient(t *testing.T) {
 	type args struct {
 		config             tokenGetter
 		appVersion         string
+		invokingAgent      string
 		logVerboseHTTP     bool
 		skipDefaultHeaders bool
 	}
@@ -155,6 +156,18 @@ func TestNewHTTPClient(t *testing.T) {
 				* Request took <duration>
 			`),
 		},
+		{
+			name: "includes invoking agent in user-agent header",
+			args: args{
+				appVersion:    "v1.2.3",
+				invokingAgent: "copilot-cli",
+			},
+			host: "github.com",
+			wantHeader: map[string][]string{
+				"user-agent": {"GitHub CLI v1.2.3 Agent/copilot-cli"},
+			},
+			wantStderr: "",
+		},
 	}
 
 	var gotReq *http.Request
@@ -169,6 +182,7 @@ func TestNewHTTPClient(t *testing.T) {
 			ios, _, _, stderr := iostreams.Test()
 			client, err := NewHTTPClient(HTTPClientOptions{
 				AppVersion:         tt.args.appVersion,
+				InvokingAgent:      tt.args.invokingAgent,
 				Config:             tt.args.config,
 				Log:                ios.ErrOut,
 				LogVerboseHTTP:     tt.args.logVerboseHTTP,