fix: use --disable-builtin-mcps for copilot sandboxing

Replace --available-tools (which doesn't effectively restrict tools)
with --disable-builtin-mcps to prevent MCP server access. Built-in
tools remain available but are mitigated by running from a temp
directory with a filtered environment and --no-ask-user.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: wesm

internal/insight/generate.go

@@ -468,7 +468,7 @@ func generateCopilot(
 		"--silent",
 		"--no-custom-instructions",
 		"--no-ask-user",
-		"--available-tools",
+		"--disable-builtin-mcps",
 	)
 	cmd.Dir = os.TempDir()
 	cmd.Env = cleanEnv()

internal/insight/generate_test.go

@@ -477,7 +477,7 @@ func TestGenerateCopilot_CLIFlags(t *testing.T) {
 		"--silent",
 		"--no-custom-instructions",
 		"--no-ask-user",
-		"--available-tools",
+		"--disable-builtin-mcps",
 	}
 	if len(args) != len(wantArgs) {
 		t.Fatalf("args = %v, want %v", args, wantArgs)